Seem not to work properly with Blender 2.90.1
I installed the add-on via "Install from file" had a success message:
Modules Installed () from '/home/vortex/Programme/blender/Plugins/Godot_Game_Tools.zip' into '/home/vortex/.config/blender/2.90/scripts/addons'
But I can not search nor enable the add on in the add on browser in blender.