Pop-Up on iCal

I keep getting a pop-up message on my desktop saying that iCal can't log into my gmail account because my password may be incorrect. I keep getting the message: "The password may have been entered incorrectly. Reenter the password for account “Gmail” on the CalDAV server “calendar.google.com." I have tried all passwords that I have used in the past but still the pop-up keeps occurring. How can I get rid of this/find out my password/change my password??? I feel so frustrated because when I try to get help from Apple (phone or email) they require the serial number, and I enter the serial number of the device (my work desktop) and I get the message that this device is linked to a separate email account so it won't allow me to contact Apple!

PLEASE HELP!!!