Katniss belongs with Peeta. Here's the reason: although Gale is the closest guy to her, almost completely understands her, and loves her just as much as she loves him [just in a different way], they don't romantically connect. Gale is aggressive and destructive just like Katniss, which is exactly the point; Katniss doesn't need another "Katniss," she needs the "dandelion in the spring" (Mockingjay, Collins). Peeta is basically her other half; he possesses the attributes that she lacks--desirable, open, persuasive through the use of nonviolence or threats.
Spoiler Alert! Highlight if you want to continue reading through my perspective on Katniss x Peeta vs Katniss vs Gale. Beware: it mentions immense spoilers from the third book. In Mockingjay, Gale basically killed Prim, Katniss's beloved sister--who was in fact her most treasured person/thing. For those of you losers who haven't read the book yet or are blatantly and purposely not reading the series, Prim died due to a Capitol hovercraft dropping bombs on her and other medical assistants in charge of helping injured children at the Capitol [they were assaulted by parachutes that they thought were filled with something good but ended up being explosives], which resulted in the death of many Capitol children due to Coin's scheme to make it seem as if President Snow killed them, which would damage his position as president. The bombs that were dropped on Prim and the other children were [most likely] created by Gale and Beetee, however they were most likely unaware of what they got themselves into when creating the bombs in District 13 when Coin commanded for their assistance throughout their days there. Eventually in the book, Gale [implicitly] feels guilty about what he has done and goes off to District 2 to assist with military work. Therefore, Gale has probably accepted the fact that Katniss will never feel the same for him due to his mistake of accidentally [basically] murdering her most cherished person.
Honestly I feel confused as to if I read that from a fanfict quote from tumblr or not, but whatever. Lol. But it's just so plausible that my mindset on Katniss definitely being with Peeta was guaranteed; even without this reasoning I'd still want Katniss and Peeta to be together, however.