I'm going to theorize (which might be wrong), but a wild Pokemon can only Dynamax or even Gigantamax only if its exposed to the dynamax phenomenon for a long period of time, I know that Snorlax in the anime just decided to Gigantamax suddenly, but I'll take that to mean Snorlax did absorb a lot of that type of energy. It would have to make sense that way given that you can't Dynamax everywhere (at least if I'm correct) why it could Gigantamax there, I'll take it to likely being anime mechanics.
Perhaps, caught Pokemon can no longer absorb the energy to trigger the reaction normally, since I don't recall an instance of that happening, it's always been with a Poke ball, right? A character didn't just send out a Pokemon and it automatically dynamaxed, right? Or did that happen? I don't know.
Anime mechanics could throwaway the need for a Pokemon to have the Gigantamax factor, it makes sense Gamefreak would impose technical limitations so that not every Pokemon could Gigantamax but could at least Dynamax, I think the anime could easily take liberties and just allow any Pokemon they want to Gigantamax.
Although to be honest I'd prefer they focus more on Mega evolutions and Z-moves, to be honest, since those can happen anywhere.
I actually wonder if Lance will mega evolve Gyarados or not.