There is spray on gasket remover products at the autoparts store, but they are also a Hella paint spripper. I have best luck with gently using a single edged razor blade. My store bought gasket remover tool is a long holder for one with a screwdriver handle, but I get better control with just the blade. Finish off with cutting a small square of green scotchbright and Gumout carb cleaner spray to get the remnants.