Gabe as an older brother headcanons:
He doesn't have siblings, so he just decided to become older brother to every kid in his neighborhood
At first this consisted mainly of babysitting them
Later he started to play with them on his own, letting them beat him at olaball (which wasn't hard, really)
The most important thing was when he started to protect them from Shuriki's guards, standing up for them and giving them time to run away (often taking beating for them)
Later he told them stories about the old Avalor, it's knights, magical creatures and making up stories about the previous royal family
When he became a true guard he had less time for them, but at least he didn't have to protect them from the other soldiers anymore
He also had more of amazing stories
He also probably started teaching them a little of the guard drills, calling them his little troops
Lots of these kids probably joined the guard when they were older, thanks to him
Because of all that he's a really good storyteller
He's also good at pretending and imagining things, which often helps to cheer people up (as we saw in Snow Place Like Home)
And you literally cannot be bored around Gabe, he always finds something to do, be it training, games or just conversation
And that's also one of the reasons he's so protective
Based on a convo we had on the server and Humble Dragon Baker's @lieutenant-amuel fantastic headcanons.
If anyone has any more ideas, please add!