After activating GMC and adding default category (2 tiers in) it generates the feed but its empty.
Activity log file states the following:
SmartStore Information: 8 : INFORMATION: Log file - Google Merchant Center feed. DateTime=2015-07-28T16:26:30.7370193Z SmartStore Error: 2 : ERROR: The method or operation is not implemented. DateTime=2015-07-28T16:26:30.8464058Z