it's important to have nice clean gasket surfaces and only use new gaskets.

i would start with a razor to help get under a gasket to lift it off.
then you can use a solvent to help break down any remaining gasket material.

a wire brush (use copper or nylon on aluminum so you dont hurt the surface) or wire wheel on a low speed drill will give you a nice clean surface.

finally wipe down surface with a rubbing alcohol so it's grease free.