GZIP and TAR are two different utilities bound by the same cause. Compressing files and making file catalogs.
To compress a file, simply gzip it:
$> gzip database_dump.sql
That will compress the database_dump.sql file, and rename it to database_dump.sql.gz. Easy, and to uncompress it:
$> gunzip database_dump.sql.gz
That will restore the uncompressed version and rename to file back to .sql
But, what if you want to do more than one, or a folder. GZIP is not the tool, however it is used.
You need to TAR the files (which creates one file with the specified files in) and then gzip it:
$> tar -cvzpf compressed_file.tgz foldername
That will compress all files in the foldername folder into a file specified, and it will GZIP it for you.
If you want to just compress a few files, then instead of using the foldername have a list of the files separated by space
$> tar -cvzpf compressed_files.tgz file1 file2 file3
Easy, so to unTar them:
$> tar -zxvf compressed_file.tgz
The above will extract the files to the current folder, if you want them somewhere else:
$> tar -C /foldername -zxvf compressed_file.tgz
The above will extract the files to the /foldername folder.
Hope that helps a little.