I'm going to ask my gf to lose weight
Why are you going to ask your gf to lose weight?
I want to live a healthy life with her.
You shouldn't tell your partner what to do. You wouldn't want her bossing you around, right?
I love her so I hope she will.
It's wonderful that you want her to get healthy, but I believe it's better to just suggest it to her instead of demanding it. She might not be very comfortable in trying yet, so why not just try to love her as she is?