I reckon Gamers is what we have always called someone who turns it on for games but not always for everything else. My older sons played with a kid who did this in football, basketball, and baseball. If you watched any practices you would wonder why he was playing, much less starting, but when the game started he was a stud. Frustrated the coaches to death but could not bench him when he knew when to turn it on.
They call my son an end of game gamer in basketball. He is averaging less than 2 points a game in the first half but has averaged 14 points in the second half of the region games this year with most of those coming in the fourth quarter. The coach has come to the point of saying now I'm counting on him to crank it up in the fourth. I don't like it because at some point he will let everybody down and the shots won't fall. But right now he is riding high and the newspaper keeps saying "Not only can he pitch and hit, but he can shoot."