This is an interesting question because had Gale been chosen, I think the shoe would've been on the other foot, so to speak, with Katniss being in the position Peeta was in. I say this because she, although it was never really stated in the book, was in love with Gale. Granted, she seemed tough but I think that was a facade and would've been easily revealed had Gale been the male tribute from District 12. Imagine if Katniss would've had to confess her love for Gale as Peeta did; decide if she would want to kill Gale or save herself as she was dead set on killing Peeta until she discovered his plan, etc. To me, it would've deadened her character a bit. I really didn't like her being "hard" although I understood why she was that way.
If Gale had been chosen, Katniss would not have had the dilemma of who to love that she carried with her throughout the books. Gale was also stronger than Peeta, and had actual hunting skills. However, Peeta had more street-smarts and people-smarts.
In short, I think one of three scenarios would have likely played out. The first would be that Gale would have died withing the first few days protecting Katniss from some harm or problem. The next would be that Gale would have failed to protect Katniss in some form or fashion and then would enact his revenge by winning the games (he was certainly capable). Another, more jaded, thought is that Gale wouldn't have been able to keep his mouth shut during the process leading up to the games and would have said something detrimental to the capital. As a result, the games makers would have had a surprise or two to put Gale in his place... probably resulting in his death.
I think Gale and Katniss would have been seen as a much more obvious target to the other contestants. Their partnership would have been more clear than the partnership between Peeta and Katniss. With Peeta as the tribute, he is able to make the others believe he is going against Katniss toward the beginning of the games. This strategy never would have worked if it had been Gale in the games. The others would have taken Gale and Katniss out as soon as they could because they would have been seen as more of a threat and a better team.
Katniss would not have learned the lessons she gathered from Peeta. Gale is far more callous and set against the capital. Peeta just wanted to be able to stay himself inspite of the games. I doubt Gale would have taken the same attitude. While Gale would certainly have been able to fend for himself as a skilled hunter, he doesn't have the same ability to think through a situation and make a plan like Peeta. Katniss learned a lot from Peeta and his attitude because he was so different from the other voice she was used to, Gale.
I don't know if the actual outcome of the games would have changed. After all, Gale would surely have been at least as competent as Peeta. However, the whole story of the two of them coming together would not have happened since Gale and Katniss already had a defined relationship. Actually, it might be that the two would not have won because the whole "falling in love" storyline wouldn't have been available and people wouldn't have been wanting them to win because of it.
she would have got together with gale at the end
Gale and Katniss will most likely be counterparts and allies,and there is no doubt that both will last through till the end as you can say they are made for each other,their attributes work well together.
However,another scenario will be that the Hunger Games organizer will be more and more interested to cause trouble and manipulate both of them,thus there is a higher chance that they will die fast and quick.
Gale and Katniss would fight over the bow, killing each other. But that is not really where the story is. What makes the two from 12 work is that they have vastly different opinions about the structure of the society, while Gale and Katniss agree that The Hunger Games is a bad thing so the conflict would not be there.
One of them might have died