See the official Google Help at 'You are attempting to open a secure connection, but the server's certificate is not trusted.' - Mobile Help
Also, there is a discussion at Google certificate errors - Page 2 - BlackBerry Forums at CrackBerry.com
that discusses the "how-to" in greater detail.
My Google cert was at the end of the list, ordered as a "w" with "www.google.com" not in the "g's".
I did it, including the Fetch Status step, and it seemed to work. On the 9700 with OS 188.8.131.523, the menu is Options/ Security Options / Advanced Security Options / Certificates.