The third of a hat-trick of new signings made by Martin Allen on the same day in late-July.
Nelson joined the Gills on a two-year deal and made his debut the following day, keeping a clean sheet, in the pre-season friendly defeat of Dartford at Princes Park.
30-year-old "Nels" has previously played for Martin Allen at both Brentford and Notts County.
The Stroud born goalkeeper played for FC Battledown, Cirencester College Football Academy, Doncaster Rovers and Hucknall Town before a £10,000 transfer took him to Brentford in February 2004.
He was sent-off late-on in his Bees debut at Brighton & HA the following month but went on to play 139 times during his time at Griffin Park, which also saw him named club captain.
After being released in the summer of 2007 he joined Leyton Orient and a year later joined Glenn Roeder's Norwich City but failed to make a first-team appearance while at Carrow Road.
After a spell with Scottish Premier League outfit Aberdeen he signed for Notts County on a two-year deal in June 2010.