I've decided to tell my long distance girlfriend to lose weight.
Why do you want to tell her to lose weight?
She has gained some weight over the past few weeks and I am not as attracted to her as I used to be.
I don't think that's a good idea at all. Telling other people to lose weight is not okay, and she'll probably be mad at you.
Whatever. Maybe she'll go on a diet.
If this is a sudden and extreme weight gain, you could approach the subject from a health standpoint. Suggest she make an appointment with her doctor, who might suggest she consult a registered dietician who could guide her towards healthier food choices. Telling her she's too fat will only alienated her from you.