I don't see it happening, I think they are both in totally different places both musically and not. Gaga seems to work with other artist based on the aesthetics of the song being made, not based on their status as a celebrity or because they are "hot" and "in".
Lana and Gaga are like water and oil. You can't mix them. They both have very different styles, voice and music. Lana avoids to talk about Gaga whenever they ask her about if there's going to be a song featuring Gaga. She doesn't like her music. She doesn't like Gaga. Gaga is a free soul, and turns pop music into art, thing that Lana can't achieve as a singer. Plus, have you heard Lana's lyrics? They are very weird, odd, and in most cases don't even rhyme or make sense. Honest truth. I'm not hating on Lana, its just that Gaga and her are VERY different. Lana has a very nice voice, but her songs are very depressing, most of the time.
I dont think it would be a good song. Lana just whines or howls over slow songs constantly. Im not saying she doesnt have a pretty voice, i just think she needs to use it differently. Anyways, Gaga sounds amazing on slow songs, but we all know the dance is where its at ;)