I had the same issue and when I issued "postconf |grep smtp|grep CA" all the variables were empty: smtp_tls_CAfile = smtp_tls_CApath = smtpd_tls_CAfile = smtpd_tls_CApath =
Here's how I fixed it;
I assume your Ubuntu install created the CA path that contains all known CAs on the internet: /etc/ssl/certs, and Equifax is there.
All you're missing in your Postfix configuration is the CApath: smtp_tls_CApath = /etc/ssl/certs
Afterwards Postfix will be able to verify smtp.gmail.com's certificate as legit CA because it has it in the /etc/ssl/certs directory.