To sync Stacks data with Google Drive service, first you need to download Google Drive App from here

After you've have configured Google drive folder on your device, assuming you currently have Stacks file saved on a local folder, you just need to move your current Workplace folder from the local folder to a Google Drive folder following the Moving a workspace guide.

After the files have been synced, to access the workspace in the other pc/mac you just need to install Stacks and follow the guide to add a new workspace, Adding a new workspace locating the google drive folder you have just synced.