These instructions will not work on Gnome 50+ (so say the Gnome devs) but this will buy you some time to make a plan.
1) Install deps: sudo pacman -S base-devel
2) Set up a path to store locally build packages:
mkdir ~/pkgbuild; cd ~/pkgbuild
3) Download the Arch package source:
pkgctl repo clone --protocol=https mutter
pkgctl repo clone --protocol=https gdm
pkgctl repo clone --protocol=https gnome-session
pkgctl repo clone --protocol=https gnome-shell
4) For mutter, and gnome-session: Within each directory, edit PKGBUILD, find local meson_options=(, add -D x11=true to the end of its list.
5) For gdm: edit PKGBUILD, find local meson_options=(, add -D x11-support=true to the end of its list.
6) Now rebuild all 4 with gnome-shell last - it needs to be rebuilt after the others have as it depends on them:
cd mutter; makepkg -si
cd ..
cd gdm; makepkg -si
cd ..
cd gnome-session; makepkg -si
cd ..
cd gnome-shell; makepkg -si
Now reboot (or log out / restart gdm), select “Gnome on Xorg” from the login screen. Voila!
Later, when you run pacman -Syu for system upgrades, look for these packages. If their versions are 49.x, let it install, then re-run step 6. You may need to use makepkg -sif (f means force) to help them complete, you also may need to manually clean up the src subdirectories within each of the 4 packages if there are build failures). This again replaces the non-X11 builds with the X11-friendly versions instead. IF the pacman -Syu run shows versions 50+, don’t upgrade, or you’ll lose X11 for good.