In Exchange 2007 how do I redirect all incoming email from a certain domain (*@gmail.com) to one email address
To point you in a direction maybe a custom transport rule like defined in this article can help? Good luck!
If you have an Edge Transport server, you can do this using a Transport Rule. As far as I can see there is no way to do this on a Hub server.
EDIT: I do apologise, I misread your question - I thought you wanted to redirect emails based on the recipients address, not the sender. This should be possible on a Hub Transport server as Joost has detailed.