The package on your system xdg-desktop-portal-gnome is marked as required by xdg-desktop-portal because it provides xdg-desktop-portal-impl that the package xdg-desktop-portal actually depends on.
Its required on your system because its the package filling the requirement on your system.
Not because xdg-desktop-portal hard depends on xdg-desktop-portal-gnome specifically for everybody.
See the output of pacman -Si xdg-desktop-portal above again.
Or the output shown here: Firefox and Gajim take 10 seconds to start - #7 by cscs