Do not reformat anything 
Although all that installing / uninstalling / deleting is not good.
It works like this: When Gimp 2.10.x is installed it,
(1) Uninstalls Gimp 2.8.x or tries to. Anything that was added after the initial install will remain.
(2) First time Gimp 2.10 is run it looks for a Gimp 2.8 user profile C:\Users\"yourname"\.gimp-2.8 and copies resources fonts / brushes / scripts / plugins over to the new Gimp 2.10 user profile C:\Users\"yourname"\AppData\Roaming\GIMP\2.10 Some of these might work some might not.
If possible try again, start with a clean sheet:
Uninstall Gimp 2.10 (do not just delete it, that leaves registry entries)
Delete the folder C:\Program Files\GIMP 2\
Delete the new Gimp 2.10 user profile C:\Users\"yourname"\AppData\Roaming\GIMP\
Rename your old Gimp 2.8 user profile to prevent copying. say C:\Users\"yourname"\.gimp-2.8-backup
Now install Gimp 2.10 Use the latest version from http://www.gimp.org Run it to create a new default user profile.
Move over any old brushes etc from the backup to the new user profile (hint: a good time for a clean up)
The gimp_gmic_qt plugin for Gimp 2.10 is different to the 2.8 version get the latest for Gimp 2.10 and install. http://www.gmic.eu
Although all that installing / uninstalling / deleting is not good.It works like this: When Gimp 2.10.x is installed it,
(1) Uninstalls Gimp 2.8.x or tries to. Anything that was added after the initial install will remain.
(2) First time Gimp 2.10 is run it looks for a Gimp 2.8 user profile C:\Users\"yourname"\.gimp-2.8 and copies resources fonts / brushes / scripts / plugins over to the new Gimp 2.10 user profile C:\Users\"yourname"\AppData\Roaming\GIMP\2.10 Some of these might work some might not.
If possible try again, start with a clean sheet:
Uninstall Gimp 2.10 (do not just delete it, that leaves registry entries)
Delete the folder C:\Program Files\GIMP 2\
Delete the new Gimp 2.10 user profile C:\Users\"yourname"\AppData\Roaming\GIMP\
Rename your old Gimp 2.8 user profile to prevent copying. say C:\Users\"yourname"\.gimp-2.8-backup
Now install Gimp 2.10 Use the latest version from http://www.gimp.org Run it to create a new default user profile.
Move over any old brushes etc from the backup to the new user profile (hint: a good time for a clean up)
The gimp_gmic_qt plugin for Gimp 2.10 is different to the 2.8 version get the latest for Gimp 2.10 and install. http://www.gmic.eu