I can't add a gmail email account to my iPhone?

I am trying to add a gmail account to my iPhone. Since Google is one of the predefined services that appears when beginning the process of adding a new email account, I assume that Apple allows gmail accounts. But when I try to add the account, I get an incorrect username or password error. I have logged into my gmail account on my computer to ensure that I was entering the correct password. So, since I CAN log into my gmail account on my computer, why can't I add it as an account on my iPhone? Thanks in advance for any help!

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