Panel 1: Gordon stands there with a goofy grin on his face because he thinks that Allyson spoke to him. She looked at him and actually said "Hi!"
Panel 2: His smile collapses when a larger boy (who has been standing behind him all this time) responds to her greeting by returning in and arranging to meet by the gym later.
This baffles me because you'd think that he'd ask who the dustball in the trucker hat is and why he's standing there drooling at her. You'd think he expects someone like Gordo to stalk Allyson or something.
Panel 3: The mystery continues when he says "Bye" and she replies "Bye, Peter!" and they continue to not take note of the goon with the staring problem. One would think that they both assume that These Things Happen, that every pretty girl ever attracts slobbering losers and there's nothing they can do about it, that's just the way things work.
Panel 4: Gordon's response to being so ignored that it's like he wasn't even there is to slump on the floor and thought-bubble that the invisible man exists.
Summary: I would dispute his choice of adjective and replace it with insufferable. This is because what he does not do and never does is look at things from her perspective. What she saw is the goober yapping about how she used to be a bowser slobbering at her while she was trying to talk to the boy she thinks is going to be the one and thinks that following her friend's directive to ignore his existence so as to not encourage him is the sound one.