“Yes gmail is fussy for eg poll for inbound mail more often than say every 10 minutes and it ignore your requests.”
Re the above and your schedule_interval value of 300 here Uncaught Server Exception TypeError: unsupported operand type(s) for //: 'unicode' and 'int' - #4 by clarkej
You may want to increase that to 600-900 seconds!?
see here post 17 and 22 JobTimeoutException: Job exceeded maximum timeout value (300 seconds) - #17 by abhinavjain