This is something we've been doing for about a solid week now, and things have progressed largely. We first got the idea from a a map named Logic_portals, made by Hurricaaane. We, however liked the idea, and wanted to do something a bit like it, but in a totally different way.
And we ended up with this!
Just press the button to see how it all works, and you can keep pressing it to change the grav field, so enjoy!
Right now its a tech demo, but we plan to come out with all this in our future maps.
So hopefully you'll find it worth while and interesting, as we have.
You can find the file under Portal Add-ons.
Next time, I'd leave the news post until AFTER your file is available. :-p