Nelson, who was the third of a hat-trick of new signings made by Martin Allen on the same day in July 2012, initially joined the Gills on a two-year deal upon his release from 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 - and Football League - debut at Brighton & Hove Albion 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. One year later he signed for Glenn Roeder's Norwich City, but failed to make a first-team appearance while at Carrow Road.
A 12-month spell at Scottish Premier League side Aberdeen followed, before he signed for Notts County on a two-year deal in June 2010.
Following the expiry of his initial two year deal at Priestfield, then club manager Peter Taylor offered him a fresh contract following some superb performances during the 2013/14 season which saw him voted the club's Player of the Year.
He was the only player to be on the field for every minute of the Gills' 2015/16 campaign, and placed third in that season's Player of the Year vote behind Bradley Dack
and Josh Wright.
Nelson signed a two-year contract extension with the club in June 2016