The CPGZ file format is a file format used with the Mac OS X Unix-based operating system. The file format itself is actually a combination of two formats, it is a CPIO file compressed with the GZ compression. If you wish to open a CPGZ file, you can do so easily, using the default decompression application that comes with every Mac computer.
Press and hold the "Ctrl" key and click on the CPGZ file you want to open.
Move your mouse cursor over the "Open With" option in the context menu that appears.
Select the "Apple Archive Utility" option from the Open With menu. The Apple Archive Utility will now open the CPGZ file.