Steps:
- apt-get install compiz-plugins-extra
- Systems Settings > CompizConfig Settings Manager > Extra WM Actions > Toggle Fullscreen (provide keybinding, such as Alt-Enter)
- I found that I needed to restart the window manager for this to start working.
Source:
http://askubuntu.com/questions/2140/is-there-a-way-to-turn-gvim-into-fullscreen-mode