Update (to old question: Does Supreme King Gate Zero's negation apply to Dark Magician the Dragon Knight after the latter has left the field?): Wait I think I get it now. The reason why I couldn't special summon DMTDK was because DMTDK was not special summoned properly, that is, fusion summoned.
If right, please skip next part.
With a Dark Magician deck, I was playing against a Blue-Eyes White Dragon deck. I couldn't use Eternal Soul to revive DMTDK, and it may have been due to DMTDK being special summoned with SKGZ.
Could Blue-Eyes Spirit Dragon have had something to do with it? I don't think it was on the field.
Deck: I used ver2.0 here but Ash Blossom & Joyous Spring instead of Maxx "C".
Programme: I used an experimental "Ygoproes" which I had foolishly tampered with for an unspeakable amount of hours to try to get the program to play an exodia quasar deck instead of a frog deck. I didn't want to say this out of looking like one of those n00bs who think that there's a bug in the programme instead of thinking there may be a ruling issue.
So, SKGZ is mainly for Rank 7 summoning as described here rather than using its ability? What other roles could it serve? I guess it can be used for Allure of Darkness.