When the question is about going to the gym right now, the things that will matter are mostly what is going on with the person being asked- for example: if they feel like working out now, if they have other things they need to do, or if they are too sick or too tired. The only things about the gym that would affect the decision about whether to go at a given time would be factors like whether it's open or whether it's too crowded at a certain hour. Whether the gym is good is the type of factor you would consider when deciding to join this gym or that gym, not when you're deciding whether you want to go to the one you've already joined at a given moment.

#1

I agree with Jonathan's answers. If you want to say that you think it would be good to go to the gym, you shouldn't use the word "it." If you say "it's very good," the meaning is "the gym is a good gym," and as Jonathan says, this sounds very weird because it has nothing to do with the question. You could say something like this: - "Shall we go to the gym now?" - "That sounds good." / "That sounds like a good idea." / "Good idea!" We can use the word "that" to refer to the suggestion that the other person made. "That sounds good" means "the suggestion of going to the gym sounds good." We don't use "it" this way, so we wouldn't say "it sounds good."

Could you tell me why 2 is not correct?