OMG! No problem! This seriously made my DAY, so thanks for asking!! :D
I’ll be honest, and say that I think the people who think that about Katniss and Gale are just giving a surface level reading of their characters. Katniss has an entire character arc in the books that’s about her learning to open up to other and not put so many walls up to keep people out. The thing is, at her core, Katniss is a lot more like Peeta than even she realizes. She’s incredibly compassionate, first willing to lay down her life for a child she barely knew, and then for Peeta. She stepped up to volunteer at the reaping out of a deep love for her sister, and she tells us how rare this kind of thing is. At one point in Catching Fire, as she’s observing and noticing how broken most of the victors are, she tells us her first instinct is to protect them. Johanna highlights this feature again when she tells Katniss that Katniss bugs her because of her ‘protector of the weak’ act, only “it’s not an act” (which makes it even worse, in Johanna’s eyes :P). Katniss, for all her tough exterior and skill at killing with a bow and arrow, is one of the most compassionate people. She’s about as maternal as it gets. The entire reason she’s built up walls in the first place is because she’s much more sensitive than she wants anybody to know she is. Because when Katniss Everdeen loves, she loves a great amount.
In my opinion, this is FAR more in line with who Peeta is than who Gale is. I’m not saying that Gale is uncaring, but he isn’t like Peeta. In Mockingjay, Katniss is far more in line with Peeta’s questioning just how far is okay to go in a war. One of my favorite parts is when they’re bombing the Nut, and all Katniss can do is stand and watching, horrified, with her hand clasped over her mouth. She also finds herself wishing that pre-hijacked Peeta were there, because she so desperately wants him to help her put words to why she’s so appalled by it.
Back the timeline up even further, though, and we see that even before the Reaping, Katniss had more in common with Peeta. Contrast the flashbacks of when she first interacted with both boys: Peeta took a beating to save her with the bread, at a time when virtually no one else in 12 was willing to help her. It’s an awful lot like Katniss being the only one to volunteer for the reaping in a way, isn’t it? However, when she first met Gale, both were incredibly suspicious of each other, both had a tit-for-a-tat system worked out, and didn’t even trust or become friends with each other for several months after their first interaction in the woods.
I also think that Suzanne Collins purposely hints at the idea that Katniss was more taken with Peeta than she realized she was. In the first book, when Katniss has returned to her room upset after they talk each other up, there’s a paragraph where she goes over all the major clues that tell us, as an audience, that Peeta has a crush on Katniss. She may not realize this, because she’s been so focused on survival her whole life, she hasn’t paid that much attention to anything else, including social cues. However, to us, the reader, it’s very clear that these actions are showing Peeta’s crush on Katniss, because they’ve very universal symptoms of having one. What’s more, in the very same paragraph, Katniss mentions observing Peeta in the same way he’s kept track of her. Katniss is saying that her actions mirror hers, even though she doesn’t understand what it’s implying about either them. But you can bet that when a writer pulls something like this, we’re being given a big hint about the character’s inner mind. ESPECIALLY when that writer is as meticulous about structure as Suzanne Collins is.
In the end, I think that Katniss and Gale were too firey for each other, and their personalities would have clashed too often to ever make it in the long-run. They may have tried dating at one point, but I don’t think it would have lasted. I do think, however, that eventually either Katniss or Peeta would have worked up the nerve to talk to the other person, and once that happened, I think it would have naturally progressed to the same end as the canon story. They’re alike where it truly matters, and that’s at the core of who they are, and what they value in life. And Katniss was always more attracted to Peeta than she was Gale—even physically. No matter what, I truly believe she would have ended up with Peeta—and would be happier with him as well.