I built this the other week.
[updated schematic 2023-01-27]
A simple pulse will trigger the circuit and it will produce a gate the duration of which you can choose using the ‘LENGTH’ potentiometer. You can also use a CV signal to modulate the gate’s duration. The GATE-input is meant for gate signals the length of which you want to change.
Obviously the trigger could be from a push botton connecting the TRIGGER-input to V+.