Resynthesizer is one plugin that works in flatpak Gimp without having to add any dependencies. Anything else and you find yourself tracking down dependencies to manually install in root.
For what it is worth, my advice: Make life easy for yourself.
Add a Gimp resources folder to your home, something easy to find than /home//.var/app/org.gimp.GIMP/config/GIMP/2.10/plug-ins
example: Make your folder(s) Add in Edit → Preferences → Folders → plug-ins
Now you can add the resynthesizer files. Get them from Gimp 2.10, Resynthesizer, Linux
Unzip, check that they are still executable (which might be your existing problem) Might look look like this:
Do they work, yes.
I gave up on flatpak when I moved to (k)ubuntu 18.04 The PPA provides Gimp 2.10.14 at the moment, more than adequate. (although 2.10.18 looks like it might be a problem for the PPA maintainer)