It would be better if it was a mutual decision...maybe before the arena they can have a talk. I'm not sure how Gale could detach his heart from Katniss and I would hate that to be Prim herself. The best way would maybe for him to somehow realize that why he does admire Katniss, it might not be romantic? It could be pretty funny to see Gale and Haymitch discussing the attributes of Katniss and Prim when they're training at the Capital and Gale realizes he and Prim might be more suited while haymitch could notice how he and Katniss are compatible. 

Of course, my posts are awesome. I mean thank you...ahem. As are yours. Thinking about the first two movies makes me ship the two pairs all over again. >___>'