I use a knife blade, or a square-ended scraper. I use the knife blade where the adhesion is not all that strong - it slips under the pieces of gasket. I use the scraper where the gasket has bonded completely to the head or cylinder, and can only come off in the form of dust.
I don't use a grinder because I'm a bit heavy-handed, and I remove too much aluminium or cast iron if I go that way.
When I've removed just about all the gasket, I scrape the surface in the normal way with the square-ended scraper. I have to resharpen it several times just to do a single cylinder mower engine. Even with all the resharpening, it only takes 3-5 minutes to clean up both surfaces.
