If you decide on starting over rather than attempting to fix it, a while back i wasted the time to figure out what packages were needed for 12.0.
It is possible to get gnucash installed on slackware without updating core packages. however, gware did not have a version out that would not conflict with the slackware versions of some packages. That would take a lot more time to figure out.
If you want to attempt to undo the damage caused by removing some gware packages, you may be able to reinstall all of gware then follow the uninstall instructions. Also... I know linux often doesn't need to be rebooted... but restarting after an install/setup may help get things on track better than you can do it on your own.
It may be that some of the mount assistants that get installed with gnome (assuming so anyway) is conflicting with something by removing a package.
On the upside of all this.. 12.2 is due to come out soon. If you want to reinstall to fix it and you have the time to spare, i would wait for that then try installing only the packages you need. I would think that the packages mentioned on the gnucash slackware page would still all be around the same versions. Just be sure the read all the directions first AND double check package versions. make sure you don't downgrade any slackware packages.
Also if you already have binaries installed, there is actually some sort of piped command you can do on a binary to find out all of the required dependencies.... I'll have to search for that again.
This will find all the needed dependencies of a binary file.
objdump -x $1 | grep NEEDED