Thanks! This addon's behavior with respect to the Gmail behavior is by design though: it allows editable areas like the email composer in Gmail to intercept keystrokes so they can implement 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