One of the powers of Gruagachan is tha tof changing shape, both in form and in size. As a MAJOR difference with standard hermetic magic, your equipment changes with you and adapts the size and form that you are changing into.
Now, this is all good and easy for a gruagach with a chain mail and a sword changing into a giant: the equipment changes with him.
Q1: is the giant sword resisted by magic resistance?
I suppose the answer is a clear "yes", but just to be sure.
Now, the same gruagach changes into a size +5 giant. it goes to the forest and uproots a tree. It labors it into a huge cudgel. This cudgel is obviously a tree, so if the giant would wield it it would avoid MR aqlltogether. Cool.
Now the gruagach changes back to human size
Q2: does the tree-sized cudgel become a branch-sized cudgel?
Q3: If "yes", is it resisted by MR?
Q4: If yes, how long is the branch-sized cudgel affected before reverting to tree-size? The remaining time of the original shapechange spell (Sun duration, for example) or forever?
Q1. Yes, the sword is under the effect of magic and thus subject to MR.
Q2. No, the cudgel was naturally tree-sized. Therefore, the abatement of the gruagachan's magic should have no effect on it. Yes, he may be carrying it on his person, but it was not subject to the original spell.
Q3. I suspect no. Accordining to RoP: Faerie effects that target a creature itself are not subject to MR, but this is contradicted in other places (eg. Succuro Fortunam) so there's a case to be made either way.
I am not sure how Grugachan shape changing works. Hermetic shapechanging, there is active magic for as long as the shape is changed. For other forms, such as the Virtues Shapeshifter and Skinchanger, people have ruled that the magic is only active during the moment of change. Like Rune Magic, since it is not affecting the subject with magic resistance, it is not resistable. Perhaps Grugachan shape changing works in a similar manner.
In canon, Shapeshifting is non-magical once the change has occurred, but not the Skinchanger Virtue, which is akin to a spell.
Gruagach shape-changing for me is more like the Hermetic process of casting a spell; and because it is more flexible than Shapeshifter or Heartbeast (not limited to a list of shapes, for a start) I would rule for my saga that it is a continuing effect and therefore subject to MR. Naturally, your saga may vary.