I know you can encounter him in free roam:
- After "Slingshot" if you're quick enough, you can catch up with him and he'll roam around like a normal NPC.
- Start "That Bitch" without stinkbombs. Gary will tell Jimmy where to get stinkbombs from, after this, he'll roam around like a normal NPC and can be knocked out with getting a mission failed.
- Halloween, after beating all errands, he and Petey will be at your side.
- The Big Prank, very hard to do, but possible, after Mr. Burton steps on the dog shit, Gary will be in the school but will most likely run into a classroom or you'll have to run away from Mr. Burton.