You need GZip (not the same as a ZIP tool), which you can find here: http://www.gzip.org
Command line is simply "gzip game.adf".
From the readme:
Note about zip vs. gzip:
The name 'gzip' was a very unfortunate choice, because zip and gzip
are two really different programs, although the actual compression and
decompression sources were written by the same persons. A different
name should have been used for gzip, but it is too late to change now.
zip is an archiver: it compresses several files into a single archive
file. gzip is a simple compressor: each file is compressed separately.
Both share the same compression and decompression code for the
'deflate' method. unzip can also decompress old zip archives
(implode, shrink and reduce methods). gunzip can also decompress files
created by compress and pack. zip 2.0 and gzip do not support
compression methods other than deflation. (zip 1.0 supports shrink and
implode). Better compression methods may be added in future versions
of gzip. zip will always stick to absolute compatibility with pkzip,
it is thus constrained by PKWare, which is a commercial company. The
gzip header format is deliberately different from that of pkzip to
avoid such a constraint.