I have Bishop here who Gavin would know if he was around the Dog Park before November 2018, and even if he wasn't I'm sure he's at least familiar with who the Officers are. Bishop has a tendency to like to get to know the people living within their walls (although letting them get to know him is another story) and he's a bit of a talker, so I could see them at least having a few conversations here and there.
Now, if Gavin's only been with the Hellhounds since 2018 he likely wouldn't know Demi - but I'm open to him knowing off her whole fiasco/journey from the Dog Park to La Quinta, then the Capitol and back to the Dog Park.