You need more information about what is happening when your server tries to deliver mail to gmail. run this command at the server console, and you should get the detail:
set config SMTPClientdebug=3
and then study the debug results in the server log the next time the router attempts to deliver mail to gmail. to turn it off, run this:
set config SMTPClientdebug=0
also, the command 'Tell Router Show Queues' will tell you what is waiting to send