As several readers pointed out, this is "I Love You" in sign language. I wish FP & Bob told me that! The thumb is out so it isn't Hook'em Horns or the Metal sign.
Bryce never confirmed anything, so you make the call.
Later in the game Harper hit a no doubter, his 2nd of the game helping him pass Griffey, and he gave another signal to the rents. Not a bad night! Video after the jump: