Your GF has to like the dog. Period. I let my dogs sleep in the bed and wouldn't have it any other way. They are my family. While GFs and BFs come and go, your dog is forever.
BUT if you must keep the little guy off the bed, you must own the bed. Standing in front of the bed when he/she tries to get up will show ownership of the bed. You can do it with anything - a toy, food, whatev. That's how dogs own things. They simply guard the object of desire. Eventually, the dog will get it but you must be patient. You must be pack leader.
If you need more info, check out the Dog Wisperer. Seriously. He rocks. Good luck!