I use 2 things, a putty knife and a honing stone.
I take the putty knife and shorten the blade to about 1 1/4 in long, then put a sharp edge on it, Works great.
After scraping, I take a dual grit hone stone and some WD40 and stone the surface.
Any remaining gasket pieces show up real quick.
Once the surface is truly clean it will take on a nice sheen.
That is all it takes to be ready for a new gasket.