The bad thing about installing other Desktop environments is they install a lot of extra packages and it is sometimes hard to find and remove all of them.
Start by running this command:
sudo apt-get purge abiword abiword-common abiword-plugin-grammar abiword-plugin-mathview alacarte argyll cups-pk-helper epiphany-browser epiphany-browser-data evolution evolution-common evolution-plugins fonts-cantarell fonts-lyx gdm gir1.2-accountsservice-1.0 gir1.2-caribou-1.0 gir1.2-clutter-gst-2.0 gir1.2-evince-3.0 gir1.2-gck-1 gir1.2-gconf-2.0 gir1.2-gcr-3 gir1.2-gdesktopenums-3.0 gir1.2-gkbd-3.0 gir1.2-gnomedesktop-3.0 gir1.2-gst-plugins-base-1.0 gir1.2-gstreamer-1.0 gir1.2-ibus-1.0 gir1.2-mutter-3.0 gir1.2-networkmanager-1.0 gir1.2-panelapplet-4.0 gir1.2-polkit-1.0 gir1.2-telepathyglib-0.12 gir1.2-telepathylogger-0.2 gir1.2-upowerglib-1.0 gir1.2-xkl-1.0 gjs glchess glines gnect gnibbles gnobots2 gnome-applets gnome-applets-data gnome-color-manager gnome-desktop-data gnome-dictionary gnome-games gnome-games-extra-data gnome-icon-theme-extras gnome-mahjongg gnome-packagekit gnome-packagekit-data gnome-panel gnome-panel-data gnome-search-tool gnome-session-fallback gnome-shell gnome-shell-common gnome-software-manager gnome-sushi gnome-tweak-tool gnome-update-viewer gnotravex gnotski gnumeric gnumeric-common gnumeric-doc gstreamer1.0-alsa gstreamer1.0-plugins-base-apps gstreamer1.0-pulseaudio gstreamer1.0-tools gtali iagno itstool libabiword-2.9 libaudit0 libcaribou-common libcaribou0 libcolamd2.7.1 libcolord-gtk1 libedataserverui-3.0-4 libevolution libgdict-1.0-6 libgdict-common libgdome2-0 libgdome2-cpp-smart0c2a libgjs0c libgoffice-0.8-8 libgoffice-0.8-8-common libgtkmathview0c2a libicc2 libimdi0 libiptcdata0 liblink-grammar4 libloudmouth1-0 libmail-spf-perl libmozjs185-1.0 libmutter0 libnetaddr-ip-perl libots0 libpst4 libtidy-0.99-0 libtracker-extract-0.14-0 libtracker-miner-0.14-0 libtracker-sparql-0.14-0 libwv-1.2-4 libytnef0 lightsoff link-grammar-dictionaries-en lp-solve mutter mutter-common packagekit packagekit-backend-aptcc packagekit-tools plymouth-theme-ubuntu-gnome-logo plymouth-theme-ubuntu-gnome-text python-cloudfiles python-packagekit quadrapassel re2c simple-scan spamassassin spamc swell-foop tracker tracker-extract tracker-gui tracker-miner-fs tracker-utils ubuntu-gnome-default-settings ubuntu-gnome-desktop xsltproc yelp-tools
It will purge most of the extra packages you installed.
Then run sudo apt-get autoremove to remove extra packages and dependencies no longer needed on your system.
You might (or might not, depending on if gdm was removed already) need to use these commands to change your login screen back to Unity's default (which is lightDM) run
sudo dpkg-reconfigure gdm
Hit enter at the prompt and then select lightdm from the options.
After that you can purge gdm with
sudo apt-get purge gdm
NOTE: I think this will remove all of Gnome, but if you find any pieces left over let me know :)
