Here's the answer from @datakick (Thank you!)
This is the process in short:
install GMC feed template into datakick
install GMC feed template from the store - you can open store in datakick using menu in upper right corner
there are two feed versions you can choose from - products only or combinations. If you use product combinations in your shop, use the later, otherwise use the first one
Choose a way to generate an XML file from template
you have two options to generate XML file
endpoint - you can create a special URL. Anytime this url is opened, fresh XML file is generated and send to the requester. This is the easiest method, that ensures that data are always fresh. However, it can be used only if the feed generation time is low (less then few seconds). If you have many products in you store, this task can take much longer, even a few minutes. Then you have to choose the second option
scheduled task - you can create a scheduled task, that will periodically generate the xml file and stores it on your server, or upload it to the ftp server. For this to work, you first need to setup CRON task - cron is an external application that opens some url on your server periodically, so we can use this event to execute our processes.
you can also generate the XML file directly from datakick module. Just open the template, an in upper right corner click on ‘more’ button. Then you can download file to your computer (the same mechanism as visiting endpoint, or generate xml file on your server - simulates scheduled task).
upload file to Google Merchant Center
That’s basically it.
After you upload your file for the first time, google will process it and it may return bunch of errors.
The most common issue is that product feed does not contain ID of your products, or IDs are duplicate. This problem is caused by the fact that datakick uses product reference code as ID when sending data to google. Prestashop doesn’t require this field to be set, or to be unique, and that may cause the issue. If you have this problem, you can either fix your data - find product with missing / duplicate reference and fix these, or change the template and use product real ID as GMC id. The second solution will work just find, however it may cause you problems later - for example, if you delete all you products from prestashop and re-import them, they will get assigned different IDs, and google will not be happy with that.