How to get my girlfriend to gain some weight?

I want my girlfriend to add on weight and I dont know how to approach the issue.

if you have substantial evidence that she may be suffering from an eating disorder or insecurities I would say something like this:

hey (so and so), I really care for you and I am concerned about your health, you used to be so beautiful and full of life when we first started dating and then you started losing weight. aside from the fact that you aren't healthy anymore, im not physically attracted to you. not that a relationship is purely physical, but I cant be with you when you look like this. it scares me.

if you have a weight fetish and are trying to make her into someone shes not:

stop lying and pretending, break up with her, and find you a hefty girl. there's websites and clubs for that. I would rather you be with someone you are truly attracted to than lie to this girl. you cant help who you like, but you can prevent yourself from messing with peoples emotions.

it all truly depends on why you want her to gain weight.