I'm going to bring up my gf's sudden weight gain
What is your reason for wanting to mention her weight gain?
She always acts like she knows next to nothing about her body
I get that you're trying to look out for her health, but people may believe you're being rude by calling her out on her weight. Have you thought about how you would feel if she began treating you differently because of your weight?
I don't care, she was all icky before I found out.
Being overweight is probably something that your girlfriend is already aware of. Odds are she is trying to do what she can to loose the weight, and if she is not than she is probably insecure about it. Treating her differently will only lower her self esteem.