Magma gang's hideout is near the Reborn nightclub. However the aqua gang should say that they are preparing to attack the Magma Gang. If not then continue with the story and check back with them later. Hope this helped. ^^
I myself have always hated grinding. I just don't like it when I've got to train a team member up from scratch for a gym. Of course it keeps me playing the game longer so that's a plus I guess.