Date of birth
Martyn joined the club on a one-year deal at the end of August 2017.
The Yorkshire born player had been training with the Mariners and penned the deal after impressing Russell Slade.
He has scored 56 goals in 379 league appearances during his 11-year career as a professional, with spells at York City, Scunthorpe United, Bristol City, Millwall, and Sheffield United.
The 31-year-old became the Mariners’ 12th summer signing.