Gale is a boy that Katniss meets while hunting in the woods and eventually befriends. She recognized him as from a brief encounter at the Hall of Justice. Both she and Gale are the eldest child in their family. Both of their fathers were killed in the same mine explosion so both were called to the Justice Building to receive a medal of honor.
Katniss first meets Gale when she inspects some of his animal traps. He thinks she might be trying to steal his catch. Gale and Katniss eventually become friends and hunting partners. Gale has feelings for Katniss that go beyond just friendship, but Katniss has yet to realize these feelings.
She and Gale depend upon one another. They help each other provide for their families and they provide support for each other in such difficult times. Gale is often outspoken about the wrongs of the capital and Katniss is frequently his sounding board. Gale even agrees to protect and provide for Katniss's family when she leaves for the games.
Gale will come to play a more prominent role in the later novels.