Always used GCZ and never had problems. For my GC ISOs, after confirming their MD5 matches a good dump, I use Wit to scrub them (is that term correct for GC ISOs?) thus saving even more space after Dolphin compresses it to GCZ. I know the uncompressed ISO won't match anymore but I don't really care...
Avell A70 MOB: Core i7-11800H, GeForce RTX 3060, 16 GB DDR4-3200, Windows 11 (Insider Preview)
ASRock Z97M OC Formula: Pentium G3258, GeForce GT 440, 16 GB DDR3-1600, Windows 10 (22H2)
ASRock Z97M OC Formula: Pentium G3258, GeForce GT 440, 16 GB DDR3-1600, Windows 10 (22H2)