First of all, read this informative site Google put together:
https://mail.google.com/support/bin/answer.py?answer=77702
I then went into my Gmail account, disabled POP, enabled IMAP.
Remove the current Gmail account from my iPhone, added a new one using "Other" as an account type.
Should be good to go
😀
P.S. I'm assuming you have no other devices (computers, phones, etc) that are using Gmail POP. If so, disabling POP will eliminate those devices ability to get your email.