Is Katniss in love with Gale Hawthorne or Peeta Mellark? I have never read the books, only seen the movies.
There are various scenes in both The Hunger Games and Catching Fire that can argue both sides, but I can't keep track of them. Who is it?
I have read all three books and seen both the films. In both media, both Gale and Peeta love Katniss but not in the same way. Peeta's love comes unconditionally, with devotion, and a complete willingness to sacrifice whereas Gale's love, I think, hinges on the fact that Gale and Katniss are comrades in the revolution against the Capitol. Another way to say it is: Peeta loves Katniss the girl whereas Gale loves Katniss the girl and the Mockingjay.
The movies and especially the accompanying advertisement materials oversell the role of Gale both as a character and as a romantic competitor. For example, his role is minuscule in the first book and the first film yet he features prominently in most posters I have seen.
In the books, it's quite clear to me that the question is not whether Katniss loves Peeta or Gale, the questions are whether she realizes that she already and deeply loves Peeta and how she may act if that realization happens. You'll learn the answers to all these in Book 3 (and most likely the 3rd and 4th films) but to avoid (more) spoiling, I'll say no further.
There's not a very simple answer to this: both love Katniss, and she makes her choice in the course of the trilogy. As of the first two books/movies, Katniss prefers Gale, but pretends to love Peeta for the sake of public opinion:
She grows to love Peeta over Gale, largely because they've been through similar experiences and he can make her feel safe again. If one of them had died, she probably would have married the other without regrets. But they both survive and can all live as free people now, so she is left with a choice...and she chooses to marry Peeta.
OK, first if you have not read Mockingjay, then I would want you to know that both Peeta and Gale are Katniss' love interest as both have qualities that she adores, e.g. Peeta can make her feel safe, whereas Gale gives her strength. While Gale is ready to slaughter anyone to ensure his and his people's safety, Peeta would rather surrender himself than to see violence on either side.
Now skip this next part if you haven't read the 3rd book.
Katniss chose Peeta over Gale hands down. This has many reasons to it.
First off, Katniss, who may be the Mokingjay and the face of the revolution, deep inside she needs emotional support. This is something Gale lacks, or in fact everyone from District 12 lacks. The need to understand someone emotionally is Peeta's USP. He understands Katniss, and this is the reason she would argue less when the decision is made by Peeta than by Gale. Gale's attitude is same as that of President Snow. Kill first, ask questions later.
Secondly, they say opposites attract! So here is Gale, her hunting partner and best friend, who is opposed to the Capitol, grown up with a negative outlook and radical thinking - basically Katniss too has most of these qualities. But then there is Peeta, always looking for a reason to be happy, searching for that silver lining even in the darkest hours and more importantly he understands what he truly wants. And since the advent of the Games, what he truly wanted was Katniss to survive. Even if it meant his death. These are the qualities Katniss craves to have, and Katniss' skill with the bow and arrow is the quality Peeta craves.
In The Hunger Games, Peeta says, "When I'm in the arena, I wanna show the Capitol and Snow that they don't own me. And that they cannot control my life!". This is something even Gale often tells her back in District 12. But it's the words from Peeta that actually gets her thinking to be the face of the revolution in all the Districts. The reason is that this sort of talk was expected from Gale or even from herself as they knew that their lives were as it was because of the Capitol's rule. But Peeta was unaffected by these sentiments even in District 12. But when Peeta says these words prior to the Games, it gets Katniss thinking of the intensity of the Capitol's rule over even the most positive and humble people. This is what melts her heart towards Peeta.
He was the reason for her becoming the Mockingjay! Without him, she would have just been a soldier in Coin's army. Another pawn in the hands of another Snow! She saw that this was what Gale had become. A pawn in the hands of Coin. But Peeta remained defiant even after being rescued by Coin. He saw early on, when he was interviewed on TV in Mockingjay, that both Snow and Coin were two sides of the same coin (pun intended!). This was what Katniss realized later, which led her to kill Coin. All of this wasn't possible without Peeta. And this is what Katniss had come to love about Peeta!