Using his sword as a lightning rod to lessen the shock of the third and strongest strike to hit him is a very clever move indeed. Ganishka using his lightning to casually silence the sorcerer with the latter just shaking off the effects shows that the old man is resistant to elemental damage.
If the old Gutts was being recruited by Ganishka, he would most likely say yes because he's intent on a revenge full of bloodlust against Griffith as the Black Swordsman but it's the new Gutts we're talking about here. After having conflicted feelings on who to choose to prioritize and gaining helpful friends he could willingly defend with his life on the line, he'd definitely reject the offer.
Seeing the battle between the Kushan demons and the Apostles makes me think that I'd rather be far away from any of them because both sides look freaky af.