I had the same error. I trolled around the help site some until I found an address to send enhancement requests to, and mentioned it would be nice to have the streamlined interface available on the 770.

They also have a list of "supported" devices that they've tested and the 770 isn't on it. In fact, all the devices on the list are phones, so that gives you an idea of how m.gmail.com is positioned. It's a young offering, though. I can't imagine it won't improve.

Mike