Gale's actions will depend on Katniss' actions but I think her biggest point will be, no matter what happens, Prim is her sister and Katniss will do anything to keep her safe and happy. And she knows Gale makes her happy so she isn't about to tell him to back off but to just be careful and take it seriously. And since time is short, plan or not, Gale may want to try and be a bit more obvious since he and Prim suck at flirting in a clever way. Hell everyone sucks at that. >__>'
But I do think Gale would want her around and to talk too of course. He'll probably try to say something more obvious but probably get in his own way. Again, I dunno! She can stay though, he might need a hug/cuddle~
And I think by now, Katniss should know things won't be weird even if they're secretly in love with each other. I'm sure she'll snag his room when he's gone and make herself cozy with a book or something. Can't kick someone out who's already made themselves at home right? XP