I uncompressed this GameCube game from .GSZ format and re-compressed it with that tool you attached Zetec, and this is the result.
(uncompressed) Buffy the Vampire Slayer - Chaos Bleeds (USA).gcm 1.35 GB (1,459,978,240 bytes)
GCZ compressed Buffy the Vampire Slayer - Chaos Bleeds (USA).gcz 1.12 GB (1,209,656,207 bytes)
DEC (Gsm?) Buffy the Vampire Slayer - Chaos Bleeds (USA).gcm.dec 1.25 GB (1,348,354,048 bytes)
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Looks like GCZ was the clear winner, at least in this example....about to try a very large (Wii) game I haven't been able to compress to WBFS (because it splits it into more than one file...) so I leave it as an ISO...we'll see what that does...