Quote:my question is how could the difference be so big if all those utilities use similar algorithm to get rid of the gibirish data that fills the disc?
They don't use a similar algorithm. Both Dolphin's compression and 7z are completely lossless while I guess gcmutility shrinks the disc by removing "unused" sectors and moving things around the FST. Additionally Dolphin's compression allows arbitrary seeks while 7z does not provide that (which is why 7z will often give better results).
If you don't get any issues with games shrinked by gcmutility, go ahead with it, but you should probably test the games after shrinking (and please, do not report bugs to the Dolphin devs if you're using such tools to reduce your games size).