Just bring up being healthy. Offer to cook for her, plan fun exciting dates that involve walking far. Take her on swimming dates.

Tell her there's a new takeout or restaurant you want to try (which exclusively sells healthier foods).

Sign you both up for some sort of nutrition lecture, class. Ask for her to come along with you as a favour.

But it honestly depends, you can encourage your gf to be healthy but it doesn't mean she's going to lose weight. If you don't find her attractive now, then it's most likely down the line you'll lose attraction for her again. And if you truly love her then you'll let her go