Not bad
I'm not much for pidgeon-holing songs into genres, but if I was to guess I would say this is ambient, reason being it is repetitive, and seems a good fit for background music, and uses atmospheric textures.
Bottom line I would have liked to see more going on with this track, either some changes in the melody, or some accompanying instruments to add more variety (some more layering would have been great). I like the melody you got going there, though it doesn't seem enough to carry the track for almost 5 minutes. Besides the beats, the only real changeup is at 3:13, and it isn't that dramatic.
All in all, this piece seems like a good foundation for something that could really go farther. I would like to hear some more instrumentals, maybe some kind of lead instrument.
Other than that, not a bad background track. I'm sure your girlfriend loved it. BTW I didn't notice the piano being out of time. Production seems good.