macOS: downloading .gz from Chrome turns into .cpgz

If you download a .gz from Chrome, and double-clicking it simply creates a .cpgz, then the downloaded file is not a .gz
Try

> file --mime-type ~/Desktop/42a3-909a-1b5bdb4a4b8b.gz
text/plain

Then simply rename it into .txt, and you will be able to open it.