Despite the accessibility issues described on the Gmail entry in the App Directory of this site, I use the Gmail app to receive instant push mail notifications on my iPhone and iPad.
An update to the Gmail app became available today, version 2.0.1 and it appears that several accessibility problems have been addressed. Although the issues with the Menu button and the menu bar on the left are still there, all buttons now appear to be correctly labeled and composing new mails seems to function correctly now! Before, it was impossible to get to the message content section while composing new mails and using Voiceover.
I would now rate this app as having some issues but being usable if you are willing to work around them, maybe an update to the Gmail entry in the directory is in order.