But we don’t ship Plymouth anymore.
It was dropped over one year ago.
About a month ago we added a gschema override to use the Garuda logo instead of Arch on the GDM login screen.
Version 1.5.8 of garuda-gnome-settings provides the gschema override (look for /usr/share/glib-2.0/schemas/zz_garuda.org.gnome.login-screen.gschema.override to confirm you have it).
If it is installed but you are not seeing the Garuda logo on the GDM login screen, you probably just need to recompile the GSettings schemas.
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
On new installations the Garuda logo should be used by default.