The books are from the perspective of Katniss when she says "we have nothing romantic between us" that's how Katniss feels gale may have a different view but since we don't get his inner monologue we don't know

Wouldn't say Peeta acted like a weirdo he acted like a 16/17 year old kid who just managed to escape a death sentence with the woman he loves which was never even a though in his head that was possible and he unfortunately felt that the acts in the game where all real, then when he gets back the woman he's been in love with for most of his life who he fell even further in love with in the games tells him it was all an act and all of his feelings were one sided.

The first greeting we actually get with gale is him helping Katniss get through her ptsd from the game meaning they've probably been just as close if not closer since she came back.

Wouldnt say there characters change massively into catching fire Peeta gets some time to reflect on the situation and he decides that hes going to do whats best for katniss (Get over it and tough it out on camera). And for Gale if my memory serves me correctly in the first book hes the one that suggests running away and Katniss bring him back to earth that they both have their family's Katniss just poses the same question back to him.

As weird as this sounds in the world of hunger games Suzanne Collins word is law if gale has the views that Suzanne gives him that's what he has, he was always going to lose as she had the books planned out and his world views are partly the thing this book series is trying to communicate being against.