I know there's some people who go 'I don't like them. But i'll be damned if they start liking someone else instead'
Gotou's main problem probably isn't him seemingly getting along with Mishima, it's how quickly he started doing it after she rejected him, which could make her believe he already had some feelings for Mishima before he invited Gotou to his place. Meaning she could through a leap of logic consider him a twotimer, which insults her pride as he obviously didn't appreciate her enough to keep on trying for her after she rejected him, which makes her jealous of Mishima because of her pride.
I'm no psychology expert, but those are feasible thoughts.
He on the other hand ? She rejected me, she has no interest in me, and i'm not about to try and wreck a relationship out of the slim chance she'll change her mind, so i'll let her be.
Also feasible thoughts, but it's interesting how different they can be despite it being the same situation.