I downloaed the gzipped tarball

https://downloads.joomla.org/cms/joomla5/5-1-2/Joomla_5-1-2-Stable-Full_Package.tar.gz?format=gz

but it seems that the file has a problem since the original tarball has been probably gzipped twice:

OK, the MD5 sum of the downloaded file is identical to that on the webpage:
  6d48da7c15be52ac7b5710b1baaa5f3f Joomla_5.1.2-Stable-Full_Package.tar.gz

The file size is 24053891 bytes and the "file" command says:
  gzip compressed data

But executing "gunzip" on that file the file size is unexpectedly only slightly increased (now 24055717 bytes) and the "file" command says still:
  gzip compressed data

But renaming the new tar file to tar.gz and executing "gunzip" again creates now as expected a much larger
file (89016320 bytes) and the "file" command says now as expected:
  POSIX tar archive (GNU)

Jens