The 21-year-old embarks on his second spell away from Stamford Bridge following a successful campaign at Stevenage in League Two last term.
After quickly establishing himself as first choice between the posts at Broadhall Way, the Chelsea Academy graduate made 47 appearances in total and kept an impressive 18 clean sheets, playing a big part in Stevenage ending the campaign with the division’s third-best defensive record despite finishing 14th.
Prior to that maiden loan move, Cumming spent the 2019/20 season as third-choice keeper at Chelsea behind Kepa Arrizabalaga and Willy Caballero, deputising on the bench on five occasions.
He also kept goal in two triumphant FA Youth Cup campaigns and performed penalty shoot-out heroics in a memorable UEFA Youth League semi-final victory against Porto in 2018.
The Winchester-born keeper joins a Gillingham side that finished four places outside the play-offs last season in England’s third tier. Their campaign begins at home to Lincoln City on Saturday 7 August.