Gliffy is a great app, and Google Drive integration is an outstanding tool. However, enabling this feature basically gives Gliffy unlimited access to any and all files. This is a no-no for me and I suspect many others that take security seriously. Thankfully Google drive has permissions that allow applications to only access the files of a particular directory, or files created by the application. With these permissions, Gliffy would have access to Gliffy files, but it wouldn't be able to view any other files in the Drive.
If trying to import something like a picture for inclusion in your graph, Web Intents would be far more effective, and far more flexible than needing access to any Google Drive files. Web Intents allow apps to act as services, and in this way, Gliffy could import a graphic from any service on that users computer, registered with that Web Intent.
Keep up the great work!