Gestures are movements in co-op that you'll learn throughout the co-op story if you play up until the end of Chamber Four. You can perform any of the gesture at any time throughout the game play with your partner by pressing (UP) on the D-Pad then using the (left stick) to choose which one you want to perform. Also there are some gestures you have to do with your partner like Rock Paper Scissors or hugging. The achievement unlocks at the end of Chamber Four you'll learn a Kung-Fu move.
Gestures:
1- Highfive
2- Waving
3- Dancing (Doing the Robot)
4- Rock Paper Scissors
5- Laughing
6- Taking off your partners head
7- Hugging
8- Kung-Fu
*Also some achievements in Co-op require you to gesture to unlock them don't forget about these:
High Five;
The High Five achievement in Portal 2 worth 6 pointsCelebrate your cooperative calibration successNarbacular Drop;
The Narbacular Drop achievement in Portal 2 worth 46 pointsPlace a portal under your co-op partner while they are gesturingAir Show;
The Air Show achievement in Portal 2 worth 31 pointsPerform 2 aerial gestures before touching the ground in co-opRock Portal Scissors
Can't Touch This;
The Can't Touch This achievement in Portal 2 worth 18 pointsDance in front of a turret blocked by a hard light bridge in co-opEmpty Gesture;
The Empty Gesture achievement in Portal 2 worth 52 pointsDrop your co-op partner in goo while they are gesturing by removing the bridge under themAsking for Trouble;
Friends list with Benefits;