At the kick, Finnick had half expected it to be Johanna. But that definitely wasn't her voice, even if it did take him an additional moment to sort out who it was.
It wasn't fair to say he didn't know Gale. But he didn't know Gale well, in the same way that he knew even Peeta or Katniss. He knew Gale knew him and Annie, and had been friendly enough the entire time that they had been here. But he didn't know how to deal with Gale's presence in the room and the fact that he was apparently adamant about doing something. (He wondered, briefly, if Katniss had sent him in, but Katniss wasn't usually that covert.)
"Yes," Finnick answered flatly, because he figured that Gale was the type to take a lack of answer as a challenge.