yeah, that is a piece of cake actually
hm, well, I am not completly certain in how GIMP does it (since I am using photoshop) but true youtube/google how to merge images/photos into one. But what i can say to you though, is that in photoshop/gimp you work with different layer, so it is quite easy to just erase everything in the image except your girlfriend, and then drag the image of her into the image of you.