Currently working in an art studio and there is a guy who is 30 years old (im 21) that ive maybe had talked to for 3 hours in total all around. Turns out hes mutuals with the new girl im dating who is an absolute dime. My gf told me that when they had brought up me and how she thought i was so cute the guy told me gf "to date anyone but me" because it would "ruin our brotherhood" that me and the guy had in the studio, and that me and said guy were "too close". After I heard this I saw said guy the next day and asked him if it was cool that me and my gf were now dating? He said it was cool and stormed off. Past few weeks has been bad, hes been super passive aggressive, unfollowed my social media, giving cold shoulder and is speaking a different language when im around. ( i live abroad). I am growing with rage and im thinking about confronting him head on. The thing is this dude has a gf, but she is clearly not as hot as mine.

TLDR: guy i work with told my gf not to date me, I asked him whats up and now hes being passive aggressive. How do i address snakey people?