Your Postfix config looks fine.
Please turn on debug mode in Postfix to debug this issue. Below are steps:
*) Send n testing email from Gmail. Postfix will log the IP address of this Gmail server.
*) Append this IP address in Postfix config file, /etc/postfix/main.cf, parameter "debug_peer_list =". Then reload (or restart) Postfix service.
*) Send a testing email from Gmail again, hope it will send out your email with the same IP address. Then you should get detailed debug message in Postfix log file. Paste it here so that others can help troubleshoot.