When installing gnome using apt it should prompt you at some point which display manager (aka login screen) you want to use. Just choose the one that's not gdm and your login screen wont change.
Generally its easy to install a new desktop. Just do "sudo apt install the_desktop_environment" and change the chosen desktop environment on your login screen when logging in (there should be some slider or button for that).
Mozilla using their limited funding to improve the last libre and independent web browser instead of the most useless and overdone ai hipster shit in existence challenge (IMPOSSIBLE) (FAILED)
Totally not related to recent events