You're spending christmas at THEIR house. You will look an ass if you go halves with your girlfriend. The most basic of manners requires that you come bearing gifts. This has nothing to do with "Christmas BS", it's just something that decent people do, like bringing a bottle of wine to dinner or a plate of something to a party.

Plus you'll get hot sex fom your grateful GF in her parent's house. That's [censored] awesome. The chance of getting caught makes it very memorable.