Comment 4 for bug 1300841

This site has a fix (it worked for me):
http://www.rubenbejar.com/2013/03/gvim-does-not-take-the-full-screen-when-maximized-in-kde/

Essentially, you can fix it like this:
1. Start GVIM
2. Right click on title bar, More Actions, Special Window Settings
3. Under Size and Position, set Obey geometry restrictions to Force/No

This should be a default setting, though, so I'm keeping the bug open. Someone more knowledgeable with how these settings are managed for the whole system should take a look though.