In The Gay Science, Nietzsche experiments with the notion of power but does not advance any systematic theory. The book contains Nietzsche's first consideration of the idea of the eternal recurrence, a concept which would become critical in his next work Thus Spoke Zarathustra and underpins much of the later works.
What if some day or night a demon were to steal after you into your loneliest loneliness and say to you: 'This life as you now live it and have lived it, you will have to live once more and innumerable times more' [...] Would you not throw yourself down and gnash your teeth and curse the demon who spoke thus? Or have you once experienced a tremendous moment when you would have answered him: 'You are a god and never have I heard anything more divine.'—[§341]
"God is dead"
Here is also the first occurrence of the famous formulation "God is dead," first in section 108.
After Buddha was dead, people showed his shadow for centuries afterwards in a cave,—an immense frightful shadow. God is dead: but as the human race is constituted, there will perhaps be caves for millenniums yet, in which people will show his shadow.—And we—we have still to overcome his shadow!—§108
Section 125 depicts the parable of the madman who is searching for God. He accuses us all of being the murderers of God. "'Where is God?' he cried; 'I will tell you. We have killed him—you and I. All of us are his murderers..."