Gravity is just a sticker they put on the frame that they get from a Taiwanese supplier. Of coarse that is exactly the same thing nearly everybody else does though some of them use Chinese frame builders. The frames are nothing special but there isn't anything wrong with them either.

The components for the money are damn good. Darts are old and dated but still way better than any suntour you would get on a $400 bike from your LBS. The drivetrain and brakes are way better than you would get for $400 from your LBS. I'm sure the wheels, seat post, handle bars, etc are heavy and nothing special but then they wouldn't be on a $400 bike from your LBS either.

Now you need to know what size bike you need and they need to have it in stock. You will also have to have a little bit of mechanical knowledge to put it together and maybe tune things a bit. Your LBS will be happy to do those things for you but paying them takes away some of the savings which is the point of going with BD in the first place.