This doesn't match any of the "certificate" issues I could find, and I certainly don't see any way of contacting gmail directly.  The message continues "Please get a new certificate containing a unique security number.  But how??

Dear Connie,
I am getting the same error. My date and time are both correct.
The only way that I can get the message to stop popping up all the time is to close the pop up window, go to another browser window and then go back to the gmail window. Then it does not come back for a long time.
By the way the pop up window reads:
"Alert
You have received an invalid certificate. Please contact the server administratr or email correspondent and give  them the following information:
Your certificate contains the same serial number as another certificate issued by the certificate authority.Please get a new certificate containing a unique serial number."