6. What does the extension ".gz" mean at the end of the data files ?

The extension ".gz" indicates the file was compressed with the utility gzip on a UNIX platform. To uncompress a file with a ".gz" extension on an Intel platform, download the file to your working directory. The file name should be renamed so the last character is the letter 'z'. Then run the command

gzip -d -N "filename.z"

See the answer to question #1.