This is the core functionality of a product I made for a customer. He has two legacy applications that only support backup using FTP servers, but he wanted to store those backups on Google Drive.

The challenging part was the upload to Google Drive while still looking like a normal standard compliant FTP server and running as Windows Service.

The core FTP server library is written as PCL. The underlying Google Drive library uses RestSharp.Portable which is also available as PCL and should allow the use in .NET Core, Windows Store, and Xamarin projects.