McDonald becomes the third senior player to exit the Globe Arena this summer following in the footsteps of Nick Fenton and Will Haining.
Shrimps boss Jim Bentley told the club's official website: "Gary has been the model professional and been terrific to work with.
"He is a great trainer with a fantastic attitude and has hardly missed a game since he has been here.
"Like Nick and Will he was a player I would have liked to have kept but he wasn't able to accept the terms we could offer and he has opted to look for another club and I hope he sorts out a deal soon."
Bentley added that he has offered new deals to Lewis Alessandra and Jordan Burrow while Danny Carlton will return for pre-season training having missed the majority of this campaign through injury.
However, Chris Holroyd will also not have his stay extended having joined Morecambe from Preston in January.
- Sports & Recreation
- Nick Fenton
- Jim Bentley