Plummer left Ashton Gate in 2011 and spent last season playing for Portimonense in Portugal.
The 22-year-old scored in Saturday's friendly against Evesham, which finished 1-1, and impressed enough to earn a further trial period.
"I just want to come down here and get some minutes under my belt," he told BBC Gloucestershire.
"Mark [Yates] talked to me when I was at City, but I wanted to go abroad and see what else was out there.
"Now that I'm back I want to get my head down and do as well as I can."