The 32-year-old appears surplus to requirements at Aston Villa and spent last season on loan at Melbourne City, scoring 14 goals in 16 appearances.
McCormack is expected to agree a loan deal with the Mariners - also joined by Usain Bolt this summer - ahead of the start of the season in October.
The Scotland international moved to Villa Park from Fulham in 2016 and still has two years left on his current deal with Steve Bruce's side.
Melbourne City - owned by the Abu Dhabi United Group that spearheaded Manchester City's transition into a European heavyweight - have brought in McCormack's Villa teammate Ritchie de Laet.