Suggestion to implement the ability to save and open the BPMN and DMN diagrams on Google Drive.
Google Drive and other Google type documents like Draw, Docs and Google Sites
saving and opening are two different things which probably require two different solutions. Please elaborate on what exactly you want to achieve to make it more clear to me.
Visit this site https://www.draw.io/
Try to connect your Google account with this application.
When you create, save, open…the application interacts directly with Google Drive whether to create, open or save a file.
With other applications like, Google Slides, could work with an add-on.