in/at the gym

[nyota]

in the gym
at the gym

Are both correct?

[billmcd]

in the gym
at the gym

Are both correct?

It depends. In general: "at"= a general location/area/could be outside of or inside depending on mutual understanding of the location e.g. I'll meet you at the gym. "in"= inside the location/ an enclosed area e.g. I'll meet you in the gym.