You created your account using your email, and you want to link your Gmail account as another method to sign in. Please see our guide below for more information.
Go into your account settings, and choose Sign-in Methods from the left sidebar options.
If your account is not yet linked, click on "here" to log in.
A pop-up will show, and you can choose the Gmail account you want to link.
Once selected, you will receive a message that it has been successfully linked.