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You downloaded a .tar.gz from a software project or received a compressed file from a Linux server. GZ is gzip compression applied to a single file — it doesn't bundle multiple files on its own, which is why it's almost always paired with TAR. The combination (.tar.gz or .tgz) is the standard archive format for the Unix world.
Gzip uses the DEFLATE algorithm — the same one inside ZIP — but applies it to a single stream rather than an archive of files. It's also how web servers compress HTML, CSS, and JavaScript for faster delivery (check your browser's network tab — most responses arrive gzip-encoded). The format was written by Jean-loup Gailly and Mark Adler in 1992 as a free replacement for the Unix compress utility.
Linux and macOS decompress .gz files natively from the command line (`gunzip file.gz` or `tar -xzf archive.tar.gz`). macOS Finder handles .tar.gz with a double-click. On Windows, 7-Zip (free) opens everything. For web development, you encounter gzip constantly — it's just invisible because your browser handles it automatically.