Ctrl+Z will undo in Gmail.
Actually, it will undo in most things. I get so used to using the features in modern applications that I forget the old keyboard shortcuts.
Ctrl+K Add a hyperlink
Ctrl+B Bold the selected text bold
Ctrl+I Italicize the selected text italic
Ctrl+U Underline the selected text
Ctrl+Backspace Delete the previous word
Ctrl+Del Delete the next word
Ctrl+F Find some text in the current document
Ctrl+Z Undo your last action