I haven't seen them anywhere, I'm afraid.
Shukaku: Pharaoh as a standalone is unlikely to be on GOG because it works just fine on any version of Windows.
? Not sure why that would stop them. A game doesn't have to have fallen out of compatibility for GOG to take it on. :)
Shukaku: Zeus and Poseidon are pretty old and I certainly don't know much about those games. Not sure if it's a classic a lot of people would play.
I'd rather play just about any of the post-Caesar 3 sequels than C3 itself, frankly. C3 is a good game, but it was the first of a series of a certain style of city-builders that continued in Pharaoh and Zeus and ended with Emperor. Caesar 3 has hitches and niggles (no official/proper roadblock, road-disrupting granaries) that were fixed in Pharaoh and especially in Pharaoh's successors.
IMO, Pharaoh is a better game if you're looking for high complexity C3-type game, and Zeus is a better game if you're looking for lower-complexity, more relaxing style. (Haven't played Emperor - would be interested in going back to it on GOG.)