@heatherhorns I know he shows up in the HG/SS Gen 2 remakes, when you go back to Kanto. He's in the gym that Giovani was previously running. Ash/Red is also around, but he's in a cave where Mewtwo was, and just silently tries to murder you if you find him.
You also meet Blue/Gary pretty early in Let's Go! Pikachu/Eevee, which is sort of a... not 100% a remake of gen 1, because there's a much older Blue there who goes, "yeah, I remember being like [new rival's name]. I sure was a dumb motherfucker back then. Well, good luck, kid!"
@Balina @heatherhorns Yes, he's the new Viridian City gym leader in GSC and HGSS
He also shows up in gen 7 as the boss of one of the modes of the battle tree, where he looks to be in his early to mid 20s (Red/Ash shows up as the boss of one of the other modes, and the third mode has both of them as a team for the boss)
-F
@Balina @heatherhorns i wouldn't be surprised if he also showed up on the gen 5 battle subway, a lot of well-known gym leaders and champions/e4 members from previous gens could show up there, and he's been both a gym leader and a champion
-F