Sorry, I can't reproduce your issues with "-comp gzip" causing mksquashfs to 
remove /bin/gzip.

Neither with:

 * Squashfs build information: running binary mksquashfs using options: -b 256k 
-comp gzip

# unsquashfs -l grml-medium-sid-i386/grml_cd/live/whiskey-ftw.squashfs | grep 
bin/gzip
squashfs-root/bin/gzip
squashfs-root/usr/lib/klibc/bin/gzip

nor with:

 * Squashfs build information: running binary mksquashfs using options: -b 256k 
-comp xz

# unsquashfs -l grml-medium-sid-i386/grml_cd/live/whiskey-ftw-lzma.squashfs | 
grep bin/gzip
squashfs-root/bin/gzip
squashfs-root/usr/lib/klibc/bin/gzip

I'm closing this bugreport therefore, please reopen the issue if you can 
reproduce your problem. Thanks!

Thanks for your patch!

regards,
-mika-