I actually used mineral spirits on the gummy residue.Try acetone. It cuts most any glue. Plastic & acetone don't play nice. Wooden popcicle stick will get it.
Tried it on the gasket and it didn't do anything to any of the subborn parts, even carefully put it on the bits to soak. It did nothing to the gasket material. The only thing it would do to anything, is take off any soft residue, but it wouldn't even soften the gasket material.
The stuff that just wasn't coming off at all with the plastic scrape, or soaking in spirits and using the wire brush... I just had to pick it off with an awl. I went at a low angle with it, scraped from the side and moved the tip side to side against the grain of the original tooling marks.
I didn't want to use anything stronger than mineral spirits because there already was chips and raised edges on the engine paint in various places. I still lost engine paint here and there (mostly due to raised edges in chipped parts), but it can be touched up.
But, it's all done now except the lifter blocks themselves.