Is your gmail account associated with an email application on your phone ?
if so, the best thing is to delete the gmail account on your phone and create a new one, rentering your gmail username and password. (i mean by that the configuration related to your gmail account, not the actual gmail account)
I have an iPhone 4 and Gmail does problems sometimes with the iOS mail application. I have to cancel the Gmail account on the mail application and create a new one for it to work again.