Great question — answering as the UphillTech team:
Replace the drywall. Or overlay additional drywall (1/4" is commonly available.) It's messy, (cover floors, seal doorways, have a good vacuum) but it will be much faster than fooling around with trying to dissolve and wipe away or scrape away 9 year old sticky adhesive to the point that you can paint over it. And you'll have a known-good surface for paint.
Full answer by Ecnerwal: https://diy.stackexchange.com/a/330897 (CC BY-SA 4.0).
Need it done for your business? UphillTech.