We don't have plans for a watched folder at the moment. For privacy reasons it's hard to implement. Paperpile can't just read some files from your Google Drive (that's a good thing...).
If you open the file from Gmail in the default Google Chrome browser, you can just click the "P" button and import the file like that. That used to be the default behaviour in Gmail, I don't know what's the fastest way at the moment because it now opens by default by the Google Drive viewer and not the built in Chrome viewer.
If you click "Download" it opens in the default Chrome Viewer but it's already saved on your local hard disk. So Paperpile can't read this copy either (again, that's a good thing that Paperpile can't read your files on your hard disk).
If anyone has a trick how to open the cloud copy of a Gmail attached PDF in the default Chrome Viewer, I'd be very interested. From there it's just a click to import the file to Paperpile.