I did an evil dark urge run,( which I abandoned halfway act 2), where I cut off Gale's hand and left him to die in his stone on the first contact. But I never had this explosion happening, even after many long rests.
Just as failing the "pull from rock" throw, what you did does not activate the Gale Go Boom! event.
Any advice for this problem or bug?
Load last save before going to bed that fateful night, go to place where Gale's dead body is polluting the environment and revive him.