GK is more of a dungeon crawler while Zodiac is more of an MORPG. The way GK feels is completely different than the way Zodiac feels. GK feels (and is) completely custom from ground up, whereas Zodiac feels like it's using Graal's default mechanics...except with custom weapons and spells. GK is deeper in that it has so much more useless **** to do, whereas Zodiac is focused on actual gameplay.

Try comparing Diablo to WoW, the comparison is similar.