The 20-year-old joined the Gunners’ Hale End academy from Chelsea in 2016 and made his debut in a Europa League group stage game against Vorskla in November 2018.
However, the centre-back did not make a single first team appearance this season, though he was in the matchday squad for the Premier League defeat to Brighton in June.
The loan move is designed to give him the opportunity play regular first team football, with Arsenal well overstocked in terms of numbers in central defence and Mikel Arteta keen to strengthen the position in terms of quality this summer.
“We are delighted that Zech has joined us,” said Gillingham boss Steve Evans. “The lad has taken his time, spoken to numerous clubs and he is a superb addition.
“I would like to acknowledge the support of Mikel Arteta and his staff at Arsenal for agreeing that Zech should continue his education here.
“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.”