AFAICT, only gravity 1 (NE) and 2 (E) actually do anything. All other gravity values just squash the notifications to the NE (upper right).
The central misunderstanding behind the decision to fix the osd bubble and not provide configurability is that you know how a user is using their desktop. E.g. since users have the choice of mail client, you can't know whether the osd obscures important real-estate during message composition. In my case, that's exactly what happens. So when osd is displayed, I'm effectively interrupted and prevented from continuing to write the email I was working on. That's a horrible productivity killer.
I fully support making opinionated decisions to give a good user experience in the default case. There must be some accommodations for customization though, or you'll really need to prevent the use of alternative editors, mail clients, web browsers, or even positioning of IM clients, etc. Either you have to lock down the entire desktop, or you have to provide people with a way to customize intrusive notifications. The alternative is to seriously negatively impact productivity for some slice of users.