I am (or at least used to be) in the same situation as your girlfriend. My mother kept telling me I need to watch my weight for the past few years. I grew up with an older brother repeating how ugly I am every chance he got. I dropped almost 10 kilos in the past year and gained a lot of muscle, but still feel ugly and fat. It just happens less often than it used to.
There is actually not much you can do to help her with that. You might want to suggest she gets some professional help. She seems to have unresolved issues with her appearence that a professional therapist could work on with her.
What you can do is give her support. Try to acknowledge the way she sees herself, instead of reponding to her by saying she is gorgeous. You can try to make her question herself if you want, but it would probably be better to not negate the experience she is having. For example, you could direct her attention to something else.
Her: Look at my belly, it's so full of fat
You: I am sorry you feel that way about it. I personally find it very cute. Actually, as much as your gorgeous shoulders.
Try to use some feature she is not insecure about when you point her attention to some other part of her body.
Also, you might want to read into body dysmorphia. Not saying she has that, but it might be a possibility, and reading about can give both of you pointers on the matter and on seeking external help.
Good luck on that journey and thank you for caring about her and wanting to help her.