*) Can you reach aspmx.l.google.com on your server? e.g. ping, host, dig, etc.
*) Can you telnet to Gmail port 25? For example:
$ telnet 74.125.25.27 25
Trying 74.125.25.27...
Connected to pa-in-f27.1e100.net.
Escape character is '^]'.
220 mx.google.com ESMTP g9si14080014pac.68 - gsmtp