I'm secretly making my gf gain weight
What do you hoping to accomplish by making your gf gain weight?
My plan is working, and it turns me on.
It's not fair to your gf to make her gain weight secretly just because you have some weird fetish. You are going to cause her to have health issues down the road.
Her body is not my concern, it's her eating habits that are.
Not being concerned about your girlfriend's body and health is not very loving. Wouldn't you feel better doing things that would keep her safe and well?