If ALL the compression parts inside the gearbox seal (so: piston head to cylinder, cylinder to cylinder head, cylinder head to nozzle in its fully extended position) then the issue is most likely around the hop up area.
Check if your nozzle to hop rubber seal is good, install the gearbox with the nozzle at its resting position (fully extended) inside the gun, then shine a light down the muzzle and check if there is any light coming through the hop unit's feed tube.
If there is, then your hop rubber is worn/torn or the nozzle is too short. Replace accordingly. If your nozzle doesn't have an o-ring, replace it regardless, make sure to measure its length with a caliper and buy accordingly.
DO.
NOT.
USE.
PTFE.
TAPE.
TO.
SEAL.
COMPRESSION.
PARTS.