Personally, I think Gallus is going to turn out to be the heir to the Griffonstone throne, he does seem set up to maybe be the main element in his group so giving him a royal position much like Twilight's. His lack of a known background would explain why he didn't know this. It would also make sense to do this with a character we are already invested in as opposed to characters who have only appeared once, twice, or not a all before. I also think it could benefit Gallus' character development as he appears to want nothing to do with Griffonstone, even more so that other griffons for understandable reasons.
One idea I think could be an interesting twist is if it turns out that Twilight and the griffons knew from the start that Gallus was the heir and that was the reason they had sent him to the school over any other griffon. Gallus doesn't appear to be related to any griffon who would support him, and he could assume (much like he does in multiple fan fictions I've read with him) that he was only sent to the school because no one in Griffonstone, but it could be something big for him if he discovered they sent him because they mean for him to eventually lead them someday. Gallus would certainly be reluctant to accept to position given his refusal to even go home.
Besides, I think it would be good look for him.