Actually, on my 2nd generation iPod Touch, but its the same app as the iPhone. I had the most annoying problem: I followed all the Gmail instructions on setting up iPhone mail for gmail, yet when I would trash messages, they would remain in my inbox. After tons of research, I finally found an apple forums post that resolved my issue.
Basically, I needed to set gmail to expunge my message immediately on the 'Settings -> Forwarding and POP/IMAP' menu. It seems that gmail thinks expunging a message means removing it from the inbox. I thought that expunging a message meant deleting it, and I don't want my messages to be deleted. It would be very helpful if the gmail team would include a note about these settings in their instructions.