For the G’Mic issue, they just have to rebuild/repackage it. As far as I understood, the code is still right (as it should, since GIMP 2.8 and 2.10 are compatible), but the package they officially provide for 2.8 won’t work out of the box.
The problem was already there during development:
See Gmic isn't recognized in Gimp 2.9.8 · Issue #113 · dtschump/gmic-community · GitHub
A lot of people still manage to have G’Mic work fine on development GIMP, but they had to rebuild it themselves or use third-party builds.
Now all you have to do is wait for G’Mic to make a new release.
P.S.: this is about G’Mic on Windows. On Linux, the G’Mic currently released officially already works out of the box.