ilu great question, I've thought about this a bit actually. Though I've tried I've never been able to see a jealous bone in Matt's body. I think he's the type to want to show off (e.g. all the public PDA) but not to gatekeep. Partially because ngl he does have intensely polyamorous vibes to me, but also because he seems like he'd be very secure in his relationships. He knows Gunwook is obsessed with him, he does notttt have to worry about anyone taking him. 😭

Speaking of, Gunwook, on the other hand, has terrorial aggression like a Chihuahua with a ritzy bone. He's young, Matthew would most likely be his first serious relationship, he doesn't know HOW the hell to act. I think this manifests in his clinginess, although it's not the only reason for it, and his ability to bring Matthew up in every conversation whether it's relevant or not (it's not.) He wants to stake claim, he needs to prove he's a good bf in a weirdly competitive way that most people grow out of eventually, sorry to say but it's puppylove.

So, to answer uh, in a situation: if by some twisted act of God, Gunwook did something to make Matthew jealous, I think he'd address those feelings within himself first and then sit Gunwook down to talk about it. I don't think he'd want to let it fester uncommunicated. If Matthew made Gunwook jealous, I think he'd blame himself and feel guilty for reading into things too much or for thinking like he owns him, and so he wouldn't say anything about it until he eventually boils over and tearfully apologizes to a very confused Seok Matthew. He'll reassure him of course but I don't see Gunwook calming down until a few years down the line at least, maybe after they get engaged he'll think "hm, maybe I'm safe."