Seriously, the Book of Mundane Beasts has nothing even close to this.
So I thought I'd hear my fellow Ars Magica players.
May I curiously ask WHY you need a gorilla?
As for stats, I would start with a size 0 troll.
Cheers,
Xavi
Mythic Dark Continent, of course!
A trip to King Solomon's Mines?
Certainly you may.
A PC magus uses a lot of MuCo(An). He did not like small apes and went for a gorilla. we play in Constantinople and he has traveled a lot in Africa.
Wouldn't a full sized gorilla be a lot larger than a human? Size +1 or even +2?
Gorilla's would be +1 size at best, 300-450 pounds on average, above that, and they're overweight.
The table on Giants in RoP:M indicates a weight of 215-465 for a +1 size (along with a height of 6'2 to 8' but gorillas are usually shorter than this).
So 0 to +1 size seems reasoable.
+2 get to 8'-10' feet tall with weights of 465-1,000 pounds.
I'd say that females (100Kg) are in the upper range of size 0, males (150-200Kg) are in the lower range of size +1.
Sounds reasonable.
What about Str, Stm, Dex, Qik?
What about qualities, V&Fs?
Some bonus to climbing I'd wager, due to the "extra hands"
High str and sta. Medium Dex. Low Qik. I would take a giant frim ROP:M and reverse engineer it to make it smaller.
Xavi
Thought I would do something useful tonight before watching the newest episode of Spartacus. I took a while longer than expected though, so I never actually got to watching anything... The good news is that we now have a Gorilla with full stats - hope you'll like it 
Gorilla (Gorilla)
(Clawed Beast - melancholic, with choleric aspect)
Characteristics: Cun +2, Per 0, Pre 0, Com -4, Str +4, Sta +2, Dex +3, Qui +0
Size: +1
Confidence Score: 1 (3)
Virtues and Flaws: Enduring Constitution, Ferocity (when provoked), Improved Characteristics x2, Reserves of Strength, Tough, Compulsions (human meat/intercourse), Wrathful
Qualities: Aggressive, Grapple, Imposing Appearance, Fast Runner, Pack Animal/Pack Leader, Skilled Climber, Thick Fur, Tough Hide, Vocal
Personality Traits: Aggressive +3, Brutal +3, Brave +2
Reputations: Man-eater (local) 2
Combat:
[tab][/tab]Teeth: Init +0, Attack +12, Defense +7, Damage +6
[tab][/tab]Grapple: Init +0, Attack +8, Defense +5, Damage n/a
Soak: +8
Fatigue Levels: OK, 0, -1, -3, -5
Wound Penalties: -1 (1-6), -3 (7-12), -5 (13-18), Incapacitated (19-24), Dead (25+)
Abilities: Brawl 5 (bite), Survival 3(jungle/mountain), Awareness 3 (food), Athletics 3 (climbing)
Natural Weapons: Teeth: Init +0, Atk +3, Def +1, Dam +1. The Gorilla’s tough hide and thick fur combine to give it a Protection of +3.
Appearance: The Gorilla is bulky beast of pure muscle, standing over five and a half feet tall and weighing up to 30 stones (420 pounds). The females are much smaller than their male (silverback) counterparts however, standing a foot shorter and weighing only half as much. Their furs vary in both size and color with black and brown being the most prominent, often with grayish or reddish foreheads. The gorilla can jump despite its bulk; it is an excellent climber - and although its knuckle-walk looks rather clumsy the gorilla is both fast and very nimble.
[tab][/tab]The gorilla lives in central Africa and the species only appear in Mythic Europe on very rare occasions. The beast is as much a myth as an actual animal and the tales of its deeds and powers vary greatly. African folklore speaks of ‘the Pongo’; sometimes a cannibalistic Shapeshifter – sometimes a human-monkey cross with magical powers. The many different accounts seem to agree that Gorillas are giant man-beasts of terrible strength and temper; and that they relish the taste of human flesh – often raiding villages to carry away captives for purposes of cannibalism or rape.
[tab][/tab]Gorillas live in groups, called troops, most often consisting of a single male pack leader, several females and their offspring (for a Pack Leader increase Communication by 1 and add Leadership 5 (gorillas)). If roused, a silverback Gorilla is a formidable opponent, attacking with great ferocity and with powerful displays including letting out great roars, thumping the ground, throwing objects and the infamous chest-beating. Running, jumping and climbing its way to the victim, the gorilla uses its long heavy arms and great strength to make a powerful hold and then bites its victim to death with its impressive mandibles.
I like it!
I'd probably lower Dexterity to +1 (Gorillas don't look particularly agile or dexterous compared to humans, at least to me).
I'd probably boost Stamina to +3 (Gorillas really look tough as nails).
I'd consider boosting Strength to +5.
I'd give leaders higher Presence, rather than Communication, and a slightly lower Leadership (maybe 3 -- Leadership 5 is the level of a wolf pack leader, and wolf packs are much better at cooperation than Gorillas, at least "mythically").
I'd also note explicitly that the smaller size of females translates mechanically to size 0 (instead of +1), probably a 1 or 2 point penalty to Strength, and maybe no Ferocity (or Ferocity when young are threathened).
A wolf pack is the paradigmatic trained group. I would be far more wary of seeing a trained gorilla pack :mrgreen:
Seconded!
spice to taste
Hmmm, what about Berserk or Fury virtues/flaws?
BTW: the changes by ezzelino, I fully agree with them.
Berserk can work certainly, Endurance of the berserkers or a CrMe effect to cause fear with its roar. I can see an effect that acts throwing you several paces away if you are hit by the paw swipe of a gorilla. Reserves of strength? ReCo effect?
I'm glad you like it 
In general I've designed the animal following the rules for doing so - as listed in RoP:MC. Also I've tried to compare it to the beast already detailed in the The Book of Mundane Beasts. Some of your suggestions below do not fit into that IMO, others might. Suit youself 
I'm not comparing them to humans - but to other animals already detalied. Personally I would deem them more dextrous than bears, thus +3 rather than +2. Gorillas do have manual dexterity unrivaled by any of the other creatures detailed. The Gorilla is capable of wielding and throwing objects, doing summersaults and performing various climbing maneouvers. The stat can be altered rather easily though, and you are more than welcome to make adjusments to my proposal, although I'm not personally convinced yet.
At first my design had a +3 Stamina actually - and I'm still not sure which one to go for. I decided to go for +2, like that of a Wild Boar, in the end though. One of the reasons being that it felt odd to add a third Improved Characteristics to the design. If you still feel like lowering Dexterity to +2, this is an easy swap though.
I'm not so sure about this one. Please note that the Gorilla has the same base Strength as the Boar, the Bear and the Lion and a higher base Strength than a Bull as it is. After making adjustments for Size these creatures end up with Strength scores between +2 and +6, as Size really does matter. Imagine a smaller than average bear (Size +1) - with roughly the same mass as the Gorilla; would the Gorilla really be that much stronger? If you're for it please remember to add an extra Improved Characteristics - to make it follow the official rules (I've tried to do that as closely as possible).
Again I've just followed the rules. Raising Presence and granting Leadership 5 is the result of the Quality called 'Pack Leader'. Not my interpretation, but as always feel free to houserule it.
I though the size thing was given, when I mentioned their halved mass. Females are obviously no larger than +0, and through this they have standard five-step Wound Categories, Strength is lowered by 2 and Quickness is increased by 1.
Ferocity (when provoked) and Wrathful not enough for you?
Please feel free to swap 1-to-1.
I do realize that the Gorilla can get really, really angry but I decided it was no more exteme than the temper of a Bear or a Bull.
I heartily agree that one could make a more 'Mythic' supernatural version of the creature, as I touched upon in the descriptive text there are many myths to build upon. I just decided that I would stat out the mundane version of the beast first, to make it more in line with Book of the Mundane Beasts. Also, I tried to comply to the request of the OP - and as I read it, Christian didn't excactly ask for spell-like powers or a Magic Might. BTW: Reserves of Strength is already in the design for the reason of impressive showing, lifting, throwing etc.
The stats posted above are for a 'basic male gorilla' (mythic in its own right). Much stronger individuals exist e.g. in troops of Gorillas of legendary renown; with those living in Magical Auras; and obviously in the Silverback Alpha Male Pack Leader.
I agree that +4 strength seems about right. This is a full 4 points above the normal human. Just as a mighty human can go up to +5 strength, we must assume that within the spread of gorillas there are likely mighty gorillas up in the +5, +6 maybe even +7 range. So +4 sounds about right.
Teeth as a primary weapon though? Don't gorillas fight with their fists as bludgeons? I know they have really nasty teeth and that the rules for beasts don't have anything like fists, so you might want to wing it (the rules for unarmed brawling wouldn't accurately reflect the sheer power of a gorilla beat down I fear).
Precisely. In the same way that individual gorillas might have the Berserk virtue. It's just not part of the basic package.
I'm glad you pointed this out. This is my biggest personal issue with the believeability of my attempt to stat out the gorilla according to the rules. It's either teeth, claws or hoofs - take your pick I've added grappling as a combat alternative, which i find highly appropriate, but as you correctly pointed out it does not reflect the beating of the gorilla.
I considered claws since the swing-the-arm-movement seemed similar to clawing, but it didn't feel right at all. I then looked at hooves as inspiration for being a weapon that would use force to repeatedly club something bloody but I left it there. In the end I went for teeth, owing to the impressive mandibles of the gorilla. The teeth of the gorilla is supposedly comparable to those of a wolf, and are thus good for both killing and eating.
Also, It fit the 'chosen myth' about gorillas eating their human counterparts. They might hunt and kill men for sport or simply for feeding; or they might do it to enact vengeance upon those who wronged it. Alternately they might be jealous of their human counterparts for their intelligence, their women and their control of fire - and thus they eat the brains of their victims to gain their knowledge.