I've used sandpaper, razors, and grinders to remove gasket material.
For an aluminum cylinder head I usually sand down as much as I can with a medium grit wet sand, then razor softly. It's easy to take chunks out with a razor, so it's not really good at doing the whol job on it's own, but it does a very good job of removing what the sandpaper is having a hard time with. The key is to use both of these tools SOFTLY. Forcing sand paper or razors to remove something only leads to BAD removal of metal. If some gunk is being a pain, use solvents as mentioned here, and try lots of angles with the razor. It will come loose eventually.
I'll use a grinder with a soft pad on some parts, even a cylinder head, but I generally don't trust myself with a grinder. It's too easy to push too hard.