If I understand your problem, you tried to remove Gnome Desktop and now you have no login screen so you cannot log in.
Not sure what command you might have used but alot of Gnome programs are a dependant of the Unity desktop.
You should still be able to log into Ubuntu using the terminal as you say you can still boot.
After booting press Ctrl+Alt+F1 this should get you to tty 1. If so enter your username and the press enter. Enter your password at the prompt note that you will not see any response to your keypress but they will still register(this is a security feature).
After you enter your password press enter and you should be greeted with a line that has your username and the computer name followed by
~$ if so you have successfully log in to Ubuntu but with not Desktop environment. We can now try to reinstall a DE to use.
Type this into the terminal
sudo apt-get install ubuntu-desktop This should install the Unity desktop as well as any dependencies that you may have deleted when you tried to remove Gnome.
If successful you should be able to restart and use your Unity Desktop. To restart type
sudo reboot into the terminal and your computer will reboot just like you had done so from the UI.
If you still get no login screen restart and log into the terminal and type
sudo apt-get install lightdm unity-greeter
I think these are installed when you install
ubuntu-deskop but I was unable to verify that. You can install them at the same time you install the desktop if you want, they will not reinstall if you already have the latest version and will give you a warning as such but will do no damage.
Note that this will only work if you still can connect to the internet, if you have borked your system so that you can no longer connect you may be able to install from the live media that you installed Ubuntu with but it may or may not work. It may still be possible to get it installed by manually setting up the connection in the terminal but that is beyond my knowledge.
Note #2 If for some reason you cannot log into Ubuntu when you press
Crtl+Alt+F1 then boot to recovery mode (press Left Shift repeatedly during boot then select recovery mode with networking) and then you should be able to follow the instructions to install ubuntu-desktop.