I am running Ubuntu 17.10.
I decided to try the I3 window manager, but do not like it.
Now I can't switch back to Gnome. The login screen does not have a place to select the window manager to use. (It used to).
There is a place to enter my password, and at the top of the screen there is a panel with an accessibility control and WiFi, etc. icons, but no place to choose the window manager.
Here is a picture of the login screen. Note that there is no selection "cog" symbol. Sorry for the ugly "screenshot", but without being logged in, I had to use my phone camera.