Ok, I have tried Gimp and g’mic under Catalina with a similar error.
I have Gimp 2.10.18 and the plugin 2.9.0 from @Carmelo_DrRaw. It is a build from Sep 2019 and the libX11 in that package is also not accepted by my Catalina.
You have to wait for an update of the gmic plugin from @Carmelo_DrRaw. I am not able to build such a package.
I am using MacPorts for gmic and Gimp with occasional local gmic updates using local git. From the beginning that is a bit cumbersome, still later on its relatively easy to follow the quick updates of @David_Tschumperle!
Still you can try the Gimp package from @partha including a rel. recent gmic plugin.