Did you enter your settings manually or allow the N900 to autoconfigure the gmail account for you? Mine is working fine with autoconfig - only had to enter my username and password.

Troubleshooting: under "advanced" configuration options for the gmail account, make sure everything looks like this. (Help for S60, but close enough.)

Note that SSL must be enabled (and the ports configured properly).

If everything is configured properly and it still doesn't work, you need to verify that your ISP isn't blocking the port. In xterm, telnet to smtp.gmail.com on port 465. If the connection times out or is immediately rejected, then the port is probably blocked.