Gale guilts her into it. She doesn't have any romantic feelings for Gale, but she knows he loves her that way. So, whenever he is hurt অথবা sad she kisses him to make him feel better. However, you'll notice afterward she wants to pretend like it never happened. Where as with Peeta there are multiple times she gets annoyed if he doesn't চুম্বন her.
Well, i think she's kinda had a crush on Gale since they met, but in the Hunger Games, her and Peeta had to pull the প্রণয় act, and at first it wasn't true, then slowly she began to really প্রণয় Peeta, but at the same time she also likes Gale, so she just never makes up her mind (Well technically she does at the end of MockingJay)
My argument is she kisses gale out of pity. At first, the kisses may have felt real, but as she gets to know peeta better she realizes she loves him which confuses her. She knows how gale feels about her, therefore, whenever he is sad she comforts him and tries to make him feel better. Her প্রণয় for Gale slowly dies down making me প্রশ্ন if she ever loved him in the first place. Katniss kisses gale only to see if she feels the spark she once felt for him. In a way, Gale does selfless actions to "earn her love" that he thinks he deserves. That includes Gale rescuing Peeta from the capital.
