A gaston is a grip technique where the climber holds an edge with the thumb pointing down and the elbow flared outward, pushing the hold sideways away from the body. The motion looks like trying to push open sliding doors with a single hand.
Gastons load the shoulder and elbow in an unusual direction — much weaker than a normal pulling grip. They appear when holds are oriented sideways or angled away from the body in a way that no other grip will work. Gastons demand strong shoulder stability and core tension; the climber must press into the hold while keeping the body rigid against the resulting outward force.
The move is named after Gaston Rébuffat, the French alpinist whose iconic 1950s climbing photos showed him in this distinctive position. Gastons are common on overhanging routes with vertical or wave-shaped holds, and almost universal in modern competition bouldering. Train them carefully — they're a frequent source of shoulder tweaks.