I found another way to reproduce this same issue. I setup a second system on a QEMU/KVM/libvirt hypervisor, and it worked fine on the built-in libvirt spice client. But if I try and enable Remote Login via GNOME (not Desktop Sharing, but full Remote Login), I get the same GDM behavior and the same stacktrace in the journal.
I found another way to reproduce this same issue. I setup a second system on a QEMU/KVM/libvirt hypervisor, and it worked fine on the built-in libvirt spice client. But if I try and enable Remote Login via GNOME (not Desktop Sharing, but full Remote Login), I get the same GDM behavior and the same stacktrace in the journal.