You'll have to change smtp port number on service mode if you use gmail.smtp.com. Port 465 (SSL required) Port 587 (TLS required).
There a couple of port 25 options for gmail as well. They have restrictions depending which option you use. If you use aspmx.l.google.com as the smtp server, you can only send to another gmail or Google Docs account. With the other options, you are limited as to the number of emails you can send each day.