I've been reading through the responses, and I don't think they are correct. You are supposed to be able to change the notification settings within the Gmail app - but on my bionic, when I go in to settnigs, I am missing all of the choices regarding notifications that should be there (you should see a whole notifications setting list, and on mine it is just blank).
I'm not sure if this is a total bionic bug, or just certain phones. I called Verizon and they called Motorola, and I was told to go back to the store to exchange.
I'm wondering if some of the other responses about just going in to settings are from people with Bionics, or people who are just assuming they know what is supposed to happen. Knowing that would help pinpoint if this is a global Gmail app issue, or only a device by device issue.