The Mariners are making preparations as they head into the Evo-Stik Premier Division season after clinching another promotion.
As well as Nicholson, forwards Jamie Holmes, Aksel Juul and Luke Sullivan have also been released.
The release of the retained list follows the arrivals of former Darlington defenders Gary Brown and Chris Hunter, who are the Mariners' only summer signings so far.
The following players have been retained by the club: Craig Baxter, Robert Briggs, Anthony Callaghan, Gavin Cogdon, Liam Connell, Carl Finnigan, Darren Lough, Lee Mason, Dillon Morse, Matty Pattison, Jon Shaw and Barrie Smith.
The club remains in discussions with Stephen Ramsey, Michael Richardson and Louis Storey while Graeme Armstrong has been placed on the transfer list.
Skipper Julio Arca retired earlier this month.