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WARNING 7-zip has the following bug since 4.45 beta (tested up to 4.58 beta):
Creating an RGZ archive will error or always produce an archive in 7z format (older versions).

NOTE: the console version doesn't suffer from this bug and works as expected

Instalation:

Put rgz.dll in "%7ZIP_INSTALL_PATH%\Formats\rgz.dll" and enjoy.

Build 2, 2008-06-09

Source
Dll compiled with 7-Zip SDK 4.57

FIX - selected compression level was being ignored

Build 1, 2008-06-03

Not released to the public.

Initial test version.