Thanks! The Gmail behavior is by design though: editable areas like the email composer in Gmail may intercept keystrokes so they can override standard word processor shortcuts.
I've thought about expanding the configuration so you can tweak exceptions and exceptions to the exceptions. Realistically, I'll probably never get around to that, so I suggest using a Greasemonkey script instead. You could stop Gmail's composer behavior with only a slight modification of this script, for example: https://gist.github.com/julfers/85fdae998ad855de17d2326e1c141cac