As someone's girlfriend myself, I'd say acknowledge both her weight gain and the fact that she's not technically overweight (I'm assuming this based on you saying she is "not fat at all", but you can look up some local statistics in your country to see for sure). To me, it would completely mess up my ability to gauge my own size if I were lied to about having gained weight. (This has happened to me and it makes it confusing to buy clothes because I have absolutely no idea what size to try on. Pants look like they'll fit fine and then they're completely wrong in the fitting room.) So tell her that yes, she has gained weight (and that's okay).

One approach to weight gain, if she really has gotten significantly bigger, is that people can be simultaneously fat and beautiful. I won't go into detail, but you can look things up. There's a world of beautiful fat ladies out there.

Another approach is to recognize that society often tells women they have to be beautiful

but that's not true! Your girlfriend isn't here to look pretty; she's here, like everyone else on this planet, to have some fun in life. So my perspective on it is that I'm not beautiful, and that's fine because I'm not here to be easy on the eyes, I'm here to play video games and go swing dancing and learn new recipes. Similarly, my body's purpose is not to appear beautiful; my body's purpose is to carry me through day-to-day things, like dancing and eating good food and moving into a new apartment. To that end, I go to the gym just to be strong enough to do what I want to do (like lift boxes into my new apartment), not so I can look good for some other person's opinion.

It might not go over well if you were to tell her this right now when she's sensitive to it ("Hey babe, yeah, you're fat and ugly, but hey, you're ugly despite being fat, not because of it! They're two separate things! And also, it's okay you're ugly! You're clearly not here to be pretty!"). But this is a mindset that has very much helped me personally over many years, and maybe you can introduce it slowly to her and explain it in a way she will understand. You know her better than any internet stranger.