I remember @Aaezealh said something about skullknight refusing to die/move on even after death we have seen guts refusing the destiny that was bestowed onto him by the threads of causality despite this guts never has died he is kinda dead according to the astral world but in the physical hes a living and breathing man making him unique in the world of berserk.The details of how what really happened 1,000 years ago is to this day unknown but gaiseric definitely died and refused his fate
Unless gaiseric was a wizard or practicer of a magic which is said that magic users can avoid the vortex of souls it could mean maybe he was a daimon and his situation is more rare/unique than guts.I know this theory is gonna get roasted hard and has flaws but it was just something i thought of over time and skullknight is simply in this condition because the armor Hanrarr made and some other missing peices to this puzzle.He asserted that Daimons were humans of the Golden Age who had become spirits acting as guardians to mortal men. That fits with what Gedflynn says about how some human heroes attained that status.
That said, not all of them are former humans,