Don’t listen to acebren, he’s just yanking your chain. The gym badges are obtained by doing a ‘favor’ for one of the staff members. If you’ll notice there’s eight gym badges, and eight staff members: @smpratte , @funmonkey54 , @japanime , @djgigabyte , @milhouse , @fourthstartcg , @woolsluk , and @reinasierpe .
As for what the favors are, it depends upon the staff member. @reinasierpe and @djgigabyte will pretty much just give theirs away, which is why almost everyone has the Boulder Badge and Cascade Badge. Thunder Badge and Rainbow Badge are held by @woolsluk and @fourthstartcg , it’s easy enough to get their badges, but you have to be willing to embarrass yourself a little, for example @woolsulk made me proclaim my love for dunsparce on discord and @forthstartcg likes to make people write ‘no rarity is the true first edition’ all over the forum. @japanime has the soul badge, which can be hard to get becacuse he’s afk often, but he’s nice enough to give you it if you can catch him. As for the Marsh Badge, Volcano Badge, and Earth Badge; don’t ask me how to get those. They’re held by @milhouse , @funmonkey54 , and @smpratte and the ‘favors’ they ask you to do are just sick, which is why I don’t have any badges from those bastards.
Edit: Just remembered I have the marsh badge… I don’t want to talk about it.