In Catching Fire, Gale plays the role of a big, fat "What if… ?" for Katniss:
Gale has a lot going for him. He's tall. He's handsome. He and Katniss understand each other because they're both from the poorest part of town. He's a great hunter and a responsible provider for his family. And he and Katniss have a long history together. Not to mention, he's clearly into her. What's not to love?
The problem is, for all of the experiences Gale and Katniss have shared, there's a big one he didn't share: the Hunger Games. Peeta did. That drove a wedge between Gale and Katniss that only seems to get wider as the books progress. The more Katniss is drawn into the Capitol's messy politics, the harder it is for Gale to follow her, and the more conflicted their relationship becomes.
It isn't just about shared experience, though. Gale and Peeta are also very different guys. Where Peeta is selfless, Gale is selfish. He wants Katniss all to himself, while Peeta is prepared to let her go if it will save her life. Consider how Katniss approaches both guys with her plan of running away. Gale is all into it until he learns that Katniss expects Peeta to go, too. In contrast, Peeta expects that Katniss would want to bring Gale along, and he's still willing to follow her. But just because Gale is selfish doesn't mean he's bad. It's kind of flattering to Katniss that he wants her all to himself, although she wouldn't be able to live with herself if they left Peeta behind.
When Katniss discovers that she will most likely have to marry Peeta and keep up the fiction of their love affair for life, things get more complicated. As a reader, it's hard to know if she's more upset about (a) not having the chance to be with Gale, or (b) not having the chance to choose between the two men. Which do you think it is?
Rebel with a Cause
Even though Gale hasn't suffered like Katniss has at the hands of the Capitol, he's more than ready to rebel. Whereas Katniss is usually wishy-washy and cautious about the opposing the Capitol, Gale is certain. He's sick of the rules, sick of watching his family go hungry, and sick of having no choice about his future. He's so firmly against the Capitol that he won't even accept a gift Katniss brings him from the region.
Gale has always been one for breaking the law. Like Katniss, he's had to poach to find food for his family, so obeying the law wasn't really an option. Unlike Katniss, though, Gale gets caught and is whipped within an inch of his life. Rather than scare him into submission, though, Gale just becomes even more anti-Capitol.
By the end of the book, Gale has officially joined the rebel cause. He's on the hovercraft with Katniss and the rebels, filling her in on how the Capitol has destroyed District 12.