yeah at the moment we don’t have a direct way to monitor for file deletions in Google Drive. I suppose there is two approaches you could attempt.
You can either regularly pull a list of all files in the folder and store that list somewhere and then compare it on a regular basis to see if any files are no longer present.
Or you could explore the Google Drive API and use the HTTP request node for it. For example, the changes resources:
I would also recommend to put this forward as a feature request here - Feature Requests - n8n Community as I can imagine there would be many users upvoting this.