Is "Goetic Magic" as described in Page 124 of "Realms of Power: The Infernal" supposed to be a Virtue, a new Ability (possible, but apparently there is no rules benefit in ever learning it above level 1), or just a subject (as seems to be the case)?
The text from page 124 makes it appear to be a Virtue, because it is learned and provides several benefits that do not reference any numerical level of Goetic Magic itself.
But Page 93 calls it both an Ability and a Virtue, and the insert in Page 82 does not include it at all. Rather, there are four Major Goetic Arts Virtues, one for each Goetic power.
My current understanding is that there is no single Virtue (or Ability) named "Goetic Magic". A Goetic Magus is someone who has acquired at least one of the four Goetic Arts Virtues (supposedly by research and study as opposed to supernatural or birth circunstances). I assume that, going by the usual pattern from the core rulebook, the corresponding Ability is learned at level 1 automatically as well. Separate Goetic Arts must be bought/learned separately.
But since Virtues are not normally bought with points after character creation, I assume the only mechanical effect of their being Major Virtues is that the whole set of four Virtues will not usually be available at character creation time. they can't be bought as such later. And since those are Major Virtues, by the RAW one can have only choose one at that time.
Or maybe I am seeing this backwards, and the only effect of having one of the Goetic Virtues is starting play with a score in the corresponding Ability? The other Goetic Arts could then be learned during character advancement normally, and whether the corresponding Virtues are or could be bought as well is a pointless question with no true consequences.
This is a reference to the Goetic Magic defined pages 114ff, of the same book.
Because they are both virtues and abilities - the virtues grant you access to the Ability of the same name.
More or less correct: The Virtues are defined RoP: Infernal, p. 82-83, 83, 83 and 87 respectively. Each explicitly states that the associated ability start at 0.
Learning Supernatural Abilities asociated with virtues you did not pay for at character creation is tricky, but doable. See ArM5, 166 for the general case.
However! For the specific, infernal case, you may want to take a look af RoP: Infernal p. 126, which covers certain aspects of learning the Goetic Arts in play, especially when used in conjunction with the traditions covered in the rest of chapter 13 of RoP: Infernal.