Your messages are likely being delivered. Check the "Logs" tab of the Mailgun Control Panel.
Do you see any entries that look like this: Routed: .*@qq.com -> qiyazhuang@gmail.com 'SUBJECT HERE'
The "Routed" prefix means that the message triggered a Route. If you're seeing this, and the next log entry is prefixed with "Delivered", the message is likely being delivered to Gmail without issue. Check your Gmail spam folder if you still don't see the messages in the inbox folder.
Disclaimer: I work for Mailgun Support. :)