Realistically, the only people who know for sure are the people who run Gmail's servers. Assuming that the Gmail software honored your request to delete the message and deleted it from the hard drive of the server immediately, there would still be the question of backups.
I would be astounded if Gmail actually did this, though. I'm not saying that Google is keeping all your deleted e-mail and reading it late at night all by itself, but it would seem much more likely that all mail arriving at Gmail is kept at least for some period of time even after the user requests that it be deleted. The period in question may be hours or it may be years, only Google can say for sure.