yes you can change your status to a green card if you marry a green card holder, however, the wait time is 2+ years, in some cases even longer than that. However, if you girlfriend becomes a US citizen, you could get your green card in a little as 5 months. Also, if you marry a green card holder, you'll have to wait 5 years after having your green card to become a US citizen, if you marry a US citizen, you only have to wait 3 years