Gtk warning when opening Gedit in terminal
Use this
(nohup gedit 2>/dev/null &)
This runs gedit in the background in a sub-shell, with nohup. Hence, the error messages are handled by nohup, and closing the terminal wouldn't affect gedit. A simpler way would be:
gedit &> /dev/null
This just discards any error messages from gedit.
I'm recommending that you not use gedit via the root user. Try and use gedit through sudo instead.
Reinstalling gedit won't help, I believe you need to undo the damage done by following the guide.