All the data comes with your own Salesforce org, and is processed on Google and saved within your Google Drive. When you create a Document Action you can set what sharing permission you want to set for new google documents generated. A5 Documents itself does not process or store any of your data from Salesforce or from your Google Drive account.
When you authorize your Google account to use within Salesforce under the A5 Documents Configuration tab, you are authorizing Salesforce with the A5 Documents app to create new and modify your Google Documents. The oauth token of access is saved in a custom setting within Salesforce. The authorization is also saved in your Google Account. If you go to Apps Connected to Your Account page in Google you can see the list of 3rd party apps you have authorized to use your Google Account and can deauthorize A5 Documents there. The access token is saved within your Salesforce org and A5 Documents does not have access to this directly. Using that Oauth token saved in your Salesforce A5 Documents using apex is able to make api calls to Google to create a copy of the template document in your google drive, then to update the document with the merge fields, then to set the sharing permission specified in the Document Action. The Google Doc is saved within your Google Drive or in any google drive folder you specify that you have write access to.
If you elected to generate another format than Google Doc like PDF, Word, then the file is generated and saved into your Salesforce org. A5 Documents never processes or retains any of your data or files. New files can be saved as Attachment or File within your Salesforce org.
We have no access to your Google User data nor do we share any of your Google User data.