I'm going to make out with my gf loudly enough for her roommate to hear
That better be a joke, why do they need to hear you two do that?
I want to see if her roommate would be jealous.
You are playing silly games and it sounds like you are interested in your girlfriends roommate. If you are playing these types of games then you are not mature enough to be in a relationship and should break up before your girlfriend gets tired of you.
But she never told me how she doesn't want me doing this with her roommate.
I still I think it's a better decision to just keep things private, what would her roommate feel if she had to put up with private sexual activity all the time? You and your girlfriend would be able to do more things too, if you were completely alone!