Date of birth
Martyn originally joined the club on a one-year deal at the end of August 2017, but the 32 year-old signed a new one-year deal during the summer of 2018.
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.
In his first year with the Mariners, Martyn made a total of 33 appearances, scoring 2 goals. Martyn signed a one-year extension in May 2018.