Love
Gale was in love with Katniss. Ever since before her Reaping he thought this. He was planning on telling her after the Reaping that day, but then stupid Bread Boy had to come in and ruin it all.
Gale watched as Katniss slowly fell in love with Bread Boy. He was powerless to tell her how he felt as he watched her fall for a baker on the screen in the square. He still remembers how it felt to see her kiss him. How stupid she was to fall in love with someone who she could never be with. At least one of them would be dead within a few days.
But then they both came home. Katniss and Peeta seemed to have grown apart. Gale hoped that perhaps Katniss had been faking in the Games, and she didn't love Peeta. Maybe she could still love him.
When she took care of him when he was whipped, his hopes grew. The look in her eyes was the same way he used to see his mother look at his father. Her eyes were filled with love.
But then, she was Reaped again for the Quarter Quell. Apparently, she was carrying Peeta's baby. That didn't seem like Katniss's style, but when Peeta mentioned the toasting, and how they did it in secret, Gale was as convinced as everybody else that she was pregnant.
Again, Gale was forced to watch as Katniss and Peeta flirted with each other and had overly romantic moments. Although, Gale was halfway glad that Peeta was there to keep Katniss away from that Finnick Odair.
When the Victors broke out of the Arena, Gale knew that the Capitol would be in District 12 very soon. Luckily, Gale was in the square, and it was easy for him to raise the alarm. He got as many people out as possible, and made sure they all had food and water. When they got to District 13, he learned that Katniss was in the Hospital. This news excited him. He was glad she was okay. He felt guilty about the rush of happiness he felt when he learned Peeta had been captured by the Capitol.
As Gale watched Katniss worry over Peeta, at first he felt only jealousy. Then, he felt anger that Peeta had put Katniss through that. Eventually though, he felt resignation. Katniss obviously loved Peeta, and if that's what made her happy, then Gale would do anything to make that happen.
Gale rescued Peeta from the Capitol, and for a few minutes he thought KAtniss would be alright. But then they discovered that Peeta had been Hijacked. Gale, along with Prim, did his best to comfort Katniss, but it was obvious that the only thing that would make her feel happy would be Peeta.
After the war ended, Gale did his best to distance himself from Katniss. Peeta made her happy, and that was all that mattered. He lived in District 7, and he made a few friends there. Eventually, one of them became more than a friend. Gale realized that what he felt for Katniss was nothing more than the same thing he had felt for Prim, or his little sister. He loved her like a sister. She needed someone kind and gentle to calm her down, and Gale couldn't give her that. Peeta could.