NO ONE has EVER believed Zeus was "God". Not even his own priests in Ancient Greece. They believed he was "a god". There is a HUGE difference; a MASSIVE, MAJOR difference. I shall explain.
First, let's talk about Zeus' backstory. (You can't answer a question like this about Zeus without going into his backstory.)
Zeus, as I'm sure you know, was the child of Cronus and Rhea. Cronus and Rhea, of course, were titans. Very powerful beings at least the equal of the gods if not even more powerful. Cronus and Rhea were both the children of Gaia (the Earth) and Ouranos (the Heavens). Gaia and Ouranos ... um ... "embraced" as the ancient texts put it and produced the titans.
Eventually Zeus led a rebellion by the gods (mostly children of titans or other gods) that overthrew the titans and the gods took over.
The point is ... not even Zeus' most devoted followers EVER believed he was anything other than a creature living within the universe. A very POWERFUL being, of course, but not omnipotent. He could not defy Fate, for instance.
Most importantly, Zeus' followers DID NOT believe FOR A MINUTE that he "created the Heavens and The Earth". ON THE CONTRARY ... Zeus was the GRANDCHILD of the Heavens and The Earth.
THAT is NOT "God" by any definition!!! It is merely "a god".
"God" (capital G) means, "That Being which brought the Universe into being by sheer force of Its Own Will." "God" means "source of all existence ... the ultimate reality ... that from which all else flows". Christians, Jews, Muslims, Sikhs ... when we say, "God", we ALL MEAN THAT BEING ... The One from Which All Existence is derived.
No one EVER believed this of Zeus (or Poseidon or Apollo or Ra or Osiris or Thor, etc). Zeus, his followers believed, was CREATED BY the universe, not the other way around.
So ... EVERYONE THAT HAS EVER LIVED has rejected the notion that Zeus is "God". Many have believed that he was "a god", but that is a whole different thing.
Edit to add: I have NO IDEA whether some being existed that called itself "Zeus" and somehow got people to worship it. Don't know ... don't care.