The youngster, who played for the under-23s last term, has been with the Gunners since arriving from Chelsea in 2016.
Medley also played in the Europa League two seasons ago.
Now the 20-year-old is set to join Gillingham for the entire upcoming season.
"We are delighted that Zech has joined us," manager Steve Evans told Gillingham's official website.
"The lad has taken his time, spoken to numerous clubs and he is a superb addition.
"He is actually a Medway boy, he went to school in Gillingham and his family still live here, in many ways he is one of our own."