... and have to add lcl/interfaces/gtk3/gtk3bindings/ to Other unit files.
You should not add library paths to "Other unit files". I know that is how you do it in Delphi but in Lazarus a package (LCL in this case) injects its paths to a project when added as a dependency.
Adding it also to "Other unit files" may cause strange errors about incompatible .ppu or similar.
I don't know why those source files are in Lazarus-CCR. I guess they should be removed. Who's project is it?