Ted, you could make a specific metal scraper to help, out of an old wood chisel.
Grind the end completely flat and square with no shape or angles on it, then radius across the blade (as if you've got it flat on the bench and put a tea cup on top to mark the radius.
Now the flat side of the chisel can be put onto the casting face and you push it along to shear off the gasket without the sharp edge digging into the aluminium, if that makes visual sense. It's how old fashioned engineering scrapers were made to make flat finishes by cutting the metal when the scraper is lifted up slightly to cut the base metal.
Sounds more awkward than reality, should allow you to clean gasket and leave surface alone.
