"Manage external tools" not popping up dialog
Solution 1:
Install, if it isn't already, gir1.2-gtksource-3.0 package:
sudo apt-get install gir1.2-gtksource-3.0
Then give your user ownership over GEdit configuration directory:
sudo chown YOUR_USERNAME /home/YOUR_USERNAME/.config/gedit
Open GEdit and now you should be able to use External tools plugin.