Hit ctrl-z to pass through keystrokes. It works with Google Reader, but I'm not 100% that it works with Gmail. I know the issue has been discussed before, and I think it was fixed, but I'm not certain.
You're right. In fact, I have my vimperator setup so that it automatically does this for Google Reader and GMail, and only for the relevant tab (i.e. if I hit "v" to view a Reader article, the pass-through turns off on the new tab). I did it with this statement in my .vimperatorrc (Linux):