You should tell her she looks great - afterall, that's what you told us. There's nothing dishonest about that. Your girlfriend is aware of her weight gain (if she is getting larger, her clothes for example, won't fit as comfortably.) Be supportive. When you are together you could take walks or be physically active (mini golf, tennis, swimming, even walking in a mall requires you to move about). You could also try talking to her about her weight and ask her how she feels about herself. How does she view herself?