And I didn't really like Gale from the get-go so I was biased, but it was pretty clear--Katniss was not going to end up with him. Gale was always about death and pretty sneaky ways of killing at that. He fit right in with the Soviety-style system in 13 and deep down probably never really saw anything he did, up to and including comign up with the kind of attack that, and never even really seems to LOVE Katniss so much as want her. Peeta just from a structural standpoint is always about LIVING. He gives Katniss 'the staff of life', he's associated in her memory with a plant (dandelion) that's both edible and a sign of spring (renewal), and he only 'loses himself' when his personality's altered by force...basically if she's going to end up with someone (and NOT ending up with someone means most likely winding up like Haymitch) there's one who'll only help affirm what she learned in the first arena (stay alive at all costs, cut down enemies, hunt) or one who can not only understand the way no one else can what the Arena was like but who also is about NOT giving in to those instincts to fight back, hurt them, etc.
Not saying Gale wasn't justified in his outlook. Just saying, that would not be someone Katniss could be with after what she's been though, which given the arc of the books makes a lot of sense-she's not the person who met Gale in the woods. (It's even spelled out.)
I have to admit, not getting a lot of the complaints I heard...the second two books make sense to me.