Cross-platform file sync utility with GUI (GPL release)

 FreeFileSync is a folder comparison and synchronization software that
 creates and manages backup copies of all your important files. Instead
 of copying every file every time, FreeFileSync determines the
 differences between a source and a target folder and transfers only the
 minimum amount of data needed. FreeFileSync is Open Source software,
 available for Windows, Linux and macOS.
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 This is the "GPL release" which uses the sources as published by the author,
 as opposed to the "FreeFileSync Donation Edition".