I soak the part in naptha in my parts washer and then scrape the top, softened layer of the gasket, which comes off easily, with a single-edge razor blade scraper; but when the going gets tough (when I reach hard gasket material), I drop it back in the naptha again for a few hours. Every time you pull it out of the naptha, a bit more of the top layer of the old gasket and gasket cement is softened. I repeat this until it is completely gone. The scraping is fairly easy this way. Maybe this would work with plain old mineral spirits, too, but I haven't tried it.
Doh! I don't know why I never thought of this before!
