I need to replace the headgaskets on a stock 502 mag efi. Can I use GM gaskets or do I need to use mercruiser?

Use FelPro Marine- not the automotive.

to be exact felpro 17048

Or look into Cometics. May be overkill for a stock app but no worries with leaks. They claim to be reusable but I have found that by the time they are removed there may be fine particles of sediment between the gasket layers.