There are many secrets with him as its been said and as well as Swain saying so. Why is he alive when all other petricite isn't what is so special with Galio that makes him able to live and do all the things he does when he fights?
he was crafted a soul.
I think Durand knew a lot more than he was letting on when he made Galio. Since with the reveal of 
and through his stories we learn that Petricite doesn't nullify magic, but instead absorbs it and eventually explodes if it gets too much. If you equate the stored magical energy as simply potential energy, it will always pile up until it reaches the breaking point and the container of Petricite breaks.
Galio was made to specifically absorb magical energy and then use it as kinetic energy to move himself around. He's the only Petricite construct that can burn away the energy his body stores. Hence, we he gets sleepy if he doesn't have any magic to absorb like in his own story. He's literally running out of energy.
Durand likely knew more about Petricite than anyone else. Remember that he didn't need to make Galio an actual statue, he could have easily made him a monolith or something else that would have been unable to move, but he specficially made him a giant gargoyle.
Aside from that there is also likely the fact that Galio has absorbed far more magic than any other chunk of petricite, so who knows maybe every giant boulder of petrcitice could become sentient if it absorbs enough magic.
He still lives because he has killed all the rest. Gate Keeper isnt an alternate skin, its his reality. Puts on Tinfoil Hat
Because Durand
maybe magic can create life/soul
