A320 First Officer reported a HYD G RSVR LO LVL ECAM at cruise. The flight crew returned to departure airport and landed.

Narrative

Aircraft X. I arrived at the gate about an hour before departure. I completed normal pre-flight duties noting no abnormalities. We departed on time and reached our cruise of FL380 with no issues. While in cruise; we received a 'HYD G RSVR LO LVL' ECAM (electronic centralized aircraft monitor). The Captain continued to fly while I completed the ECAM actions. We turned the Green Pump and PTO (power takeoff) off in accordance with the ECAM. Looking in the COM (communications); it became evident that we would need to manually extend the landing gear. We used to ACARS system to advise Dispatch and Maintenance Control of the situation. They recommended that we return to ZZZ. While returning; I remembered seeing an entry in the logbook of service to the hydraulic system the night before. I looked at the entry again and it showed that the green hydraulic accumulator had been replaced. A longer than normal final approach was requested just to give us time to manually extend the gear and because the slats/flaps would be slow. We landed uneventfully without requesting priority handling and taxied to the gate. While setting up a replacement airplane for the flight; maintenance personal advised they believed that the mechanic the night before had cross threaded the fittings on the accumulator.