I want to see my girlfriend get gangbang or have threesome with two guys. Is this normal for me to get aroused and excitted?

Yes, it's totally normal.

My gf is a dominatrix, and I'm a submissive bisexual. She love to watch me get gang banged by a bunch of skaters, and we arrange a session about once a week. Once I sucked off eleven skaters in front of her. It paid off when we got home. We had the wildest kinkiest BDSM session we had ever had.

I love getting gang banged, but most of all, I love my GF for getting turned on by it, too.