If you can find a source that proves Activision owns the IP rights to Pharaoh/Zeus/Caesar 3, please provide it and prove me wrong.
I'm not sure why you're even asking. Just visit the GOG store page of any of these games, they clearly list Activision as the publisher, and have Activision copyright statements in the bottom.
All of these games were published by Sierra. Sierra was merged into Activision Blizzard in 2008. All the Sierra IP has belonged to Activision for almost 10 years now. If you want more proof, just visit sierra.com and see what it says in the bottom: "© 2016 Activision Publishing, Inc."
How would Activision be able to update the GOG versions of the games if they didn't own the rights to them?
Anyway, I doubt that this is going to have any effect on how many people come to these forums. One, I don't think many people use in-game web links, and two, the links only ever pointed to the GOG website for people who already bought the game here in the first place, so they'd be well aware of where to go for support.