Poku came through the youth ranks, playing for the under-18's last season before being promoted into the first team. He made five appearances for the Blues last season.
His first team debut was away at Waltham Forest in the 1-0 League Cup defeat in September 2010, replacing Emiel Aiken as a sub.
The youngster follows Danny Hearty who recently moved to Bowers & Pitsea, and joins up with a contingent of ex-Grays players and staff.