Thanks for replying. I was following this lead from the guide Charly, but there is nothing there
There 100% is not a gym in the Grotto. There's nothing in the Grotto. Good time...quick game :P
...? The gym is exactly where Charly says. On ground level and turn left at the fork, not going upstairs, not going to the crates.
Well; finally it appeared, but only after ingame day three. Maybe it depended on another progress?
Every quest has a description.