The “download” app feature is not meant to open up a preview of the document automatically after you generate document (screen flashes to let you know a download happened but it does not auto open document in another window). The download feature is only to download a copy of the document to your computer, which would show in your “downloads” folder. So you would need to open your downloads folder on computer in order to see it. The only way to bypass this step is you have another setting on your browser to automatically open the download document or using “Preview” program on a MAC to auto open downloads or can configure Chrome browser by inserting the ‘rs doc link’ in the ‘allow pop-ups’ page.
Another option is to access the document within Salesforce by using document action is to “attach file to record” which you could then access the document under Notes & Attachments or maybe files if you using files instead. Or you can use the “save URL link to Salesforce” feature and have a URL link to google doc right there from same window (users would need to have access to open link by your sharing settings in document action and access to your google drive that documents are stored on AND you would not be able to use app feature to “delete google doc after merge” in document action). Or you could configure email feature in document action to then send generated document to an email outside of Salesforce.