flatpak -u override --filesystem=xdg-config/gtk-3.0
flatpak -u override --filesystem=xdg-config/gtk-4.0
Source: flatpak/flatpak#3901 (comment)
flatpak -u override --filesystem=xdg-config/gtk-3.0
flatpak -u override --filesystem=xdg-config/gtk-4.0
Source: flatpak/flatpak#3901 (comment)