InnoSetup as a way to create an installer to install the games on Windows. Daniel Scharrer has created innoextract to allow the unpacking of those archives on a non-Windows platform.
The website provides information on using innoextract, but this information from the page is very useful:
GOG.com installers with a 2.x.x version number on the download page or in the filename use InnoSetup 5.5.0 and cannot be extracted by innoextract 1.2 and older. Older installers use InnoSetup 5.2.3 and usually have no version in the filename.
Some GOG.com multi-part installers with version 2.1.x or higher use RAR archives (renamed to .bin) to store the game data. These files are not part of the InnoSetup installer and require unrar or unar to be installed, as well as innoextract 1.5 or newer and use of the --gog option.
A Windows only version has more features; it is also Open Source.
MORE ARCHIVE UNPACKERS
1. unshield can be used to extract cab files from InstallShield installers and cabextract can be used to extract files from cab files. msitools can be used to inspect and build Windows Installer (.msi) files.
2. fsarchiver is a system tool that allows you to save the contents of a file-system to a compressed archive file. The file-system can be restored on a partition which has a different size and it can be restored on a different file-system. Unlike tar/dar, FSArchiver also creates the file-system when it extracts the data to partitions. Everything is checksummed in the archive in order to protect the data. If the archive is corrupt, you just loose the current file, not the whole archive.
3. makeself is a packaging tool used to make installers. It is a small shell script that generates a self-extractable compressed tar archive from a directory. The resulting file appears as a shell script (many of those have a .run suffix), and can be launched as is. Here's a HOWTO.
4. With UT2003 update 2199, a umod unpacker function was included with Unreal Tournament 2003.
Added a "umodunpack" commandlet to ucc, to extract ut2mod files.
./ucc-bin umodunpack -x file.ut2mod -nohomedir
to list contents of file.
./ucc-bin umodunpack -l file.ut2mod -nohomedir