Apologies if you have been over this. I searched the forums and could find no threads discussing it, and I haven't been over it, so here we are.
You all have my sympathies then for being taught incorrectly in school. Gallus's is not in any way wrong. Neither is Gallus'. This is a matter more of style than of grammar. If your teachers marked you down for it, then it was because they wanted you to go with one style over the other. If they taught you it was incorrect grammar, well, again, you're learning now that it isn't.
Also, by way of explanation the plural versus singular does have something to do with it. For example, if you have 1 boy, and he has a ball, you say "The boy's ball."
If you have more than 1 boy, and they have a ball, you say "The boys' ball." This is not a matter of style, but of correct grammar and communication.
For names, the same is true. If you are referring to Robert's ball, you mean 1 guy. If you say Roberts' ball, you are talking about more than one Robert.
But if the name ends in an s, that's where the style part now comes in. People have written both 's and s' for the plural of names ending in an S. This is because for written communication, writing "Roberts' " by itself is no longer clear. Do you mean multiple boys named Robert, or do you mean a family of people? Or perhaps you have one person whose last name is Roberts. So both forms of the possessive were written, and now both are accepted as correct.
For example, the Chicago Manual of Style and the Associated Press Stylebook both weigh in as authorities on this subject, and they have said and maintain the different styles over the years, and they also commonly disagree. Here is a reference page for you that compares the two: http://apvschicago.com/2011/06/apostrophe-s-vs-apostrophe-forming.html
There is also the classic Strunk and White's Elements of Style, that offers its opinion, favoring 's, as in "Charles's." Style commonly arises from usage. AP, for example, offered their style guide for newsprint and thus space was at a premium, so favoring Charles' was better because it saved space. Wikipedia discusses all this on their talk page on possessives, since it has many editors and they try to come to some agreement.
So, the upshot is, if you were taught that one style is wrong, then you were taught something wrong. It's a style thing now, not a point of grammar. Style can't be wrong, really, even though people may have vehement disagreement over it.
If you folks at USSEP want to go with the opposite style over what the writers of Skyrim chose, you are of course free to do that, but you should at least be aware that you are incorrectly labeling these as "bugs". The possessive is just in a different style. In my opinion, you are wasting time and energy correcting something that doesn't need it, and that could be better spent fixing actual errors in the game, both textual and otherwise.