IMO SC wrote to many K/P scenes and too few K/G ones for the former not to be endgame, not unless Peeta died or something. What' the point of dedicating so many pages to building up their relationship if it's going to turn out to be for nothing?
There are several indications that Katniss was going to end up with Peeta. She herself said at the end of MJ that she always knew that this was going to happen. So I believe that even if Prim hadn't died, Kat would have chosen Peeta eventually, it would have just taken longer because of the guilt she felt concerning Gale and SC sped up the process by ridding Katniss of that guilt. Katniss became an emotional wreck when Peeta was taken by the Capitol and didn't even remember her other loved ones until Gale appeared right in front of her. She remained an emotional wreck while Peeta was being held prisoner and while he hated her and didn't seem to want to live anymore. Kat's main focus was killing Snow then dying herself and even in the beginning of the book she wanted to run away from 13 and never return (away from everything, including Gale) but the only thing stopping her was her need to know if Peeta was alive. In the last chapter of MJ she contemplated taking her life but was waiting for something, that being Peeta. This is one of the many examples of how Peeta gives her hope and keeps her going, by the end of CF it's clear that she can't function well without him. Notice how Katniss was relieved that Gale wasn't making passes at her but after Peeta had been rescued, she was looking forward to his kisses lol. It was only when kissing Peeta that she felt any sort of lust or need. Although she seems quite possessive of Gale in CF (this isn't abnormal when it comes to girls and their best [guy] friends), she doesn't really seem attracted to him.
I'll also add that Gale is a bit to jealous and possessive, getting jealous of Darius and Finnick whereas Peeta is a lot more chill. He was amused when Finnick and Chaff were messing with Katniss at the QQ and I'm pretty sure Gale would have been upset if he had been in Peeta's place. It's as if Gale sort sees Katniss as a possession or something.