As a beginner-intermediate learner, my first reading of the (partial) sentence was along the lines of "it wouldn't be good to gain weight [and]..." but the given translation has an almost opposite meaning with "it's not just that it would be good to gain weight...".
I've been wrestling with this for a while and I think I've gotten my bearings somewhat. The two things confusing me are (1) the positive meaning, and (2) the "not just".
Is it right to say:
(1) it's positive because it uses the "いいじゃない" structure rather than a negatively conjugated "よくない"? It kind of got hazy for me because of the conversational 'ってこと' inserted, but I believe you can essentially ignore that aspect.
(2) is the "just" essentially because of the "なくて"?
So another way to parse the ending of the sentence is something like "wouldn't it and..." (because of the rhetorical negative and the -て continuation), but in context of the "it would be good to gain weight" base you can collapse it into "not just" getting you the final meaning?
I feel like I'm close but I'm not quite correctly parsing out the nuanced meaning of the sentence. Like I'm 80% there. Or maybe I'm totally off!