IDK maybe the GP was trying to encourage you (maybe poorly?) It is tough to watch what we eat when we are on medications that make us want to eat..and when we are in emotional states that keep us from realizing it.

Does the medicine you're on say it causes weight gain? Well, guess that doesn't really matter, since ppl can react in different ways to medicines.

I'm sorry the GP didn't seem supportive. I am having the same kind of issue with my MD.