What is this article How to install the Rust Cosmic Desktop environment on Pop!_OS?
Either, if you want to use the new COSMIC login screen, you can install the cosmic-greeter package:
sudo apt install cosmic-greeter
Once that is installed, you should be able to switch back and forth between cosmic-greeter and gdm3 with:
sudo dpkg-reconfigure gdm3
and selecting whichever login manager you wish to use.