Gas in quick disconnect QD with barb for gas line.
Barb is 1/4".  

1/4" or 5/16" beverage tubing can be used in place of gas line.

FYI- Sometimes ball lock fittings are a challenge to quick disconnect "on and off" a cornelius keg. We recommend spraying the body connect on the keg first with a sanitizer solution which acts as a lubricant. When applying the ball lock fitting to the body connect, give a twist and turn to make it easier to connect.