Is there a keyboard shortcut that will sign me out of GMail?
Unfortunately, there isn't a shortcut to use in Gmail for signing out.
But you can copy the sign out link
Assuming you are using Google Chrome, use ClickLess, a Chrome extension, to assign a shortcut for the link of sign out, which is mentioned above.
Go to the options of this extension and add the shortcut.
Now whenever you need to sign out. Use Ctrl+Alt+S, a small windows will pop out, press now s which is the shortcut key for signing out from Gmail.
If you use Opera, add the above mentioned URI to one of your speed dials (first 9). Then use Ctrl+N (where N = digit 1-9) when you are on Gmail to sign out.
I added this on my 1st speed dial, so now I can easily sign out by hitting Ctrl+1.