There's a mod called Classical World that requires both this game and Hegemony Rome and all their DLCs. Is there a way to make it run with GOG's Hegemony III?

Thanks for your answer, but the problem is not about getting the mod itself. The mod says it requires Hegemony Rome, but GOG doesn't sell Hegemony Rome. I don't mind if I have to buy it on Steam, but I would prefer not to purchase Hegemony III on Steam because I've already bought it from GOG.

But I guess that's the only possible way.

Hey, I think I've read that the mod actually is working even without Hegemony Rome. Hegemony Rome is only required for the special unit graphics. So there may be an occasional error message when the mod tries to load these, but you can close the message with F2 and it should be fine. Not tested this myself, but read this on the reddit r/hegemony_series from a guy who contributed to the mod.

Actually, in ModDB I've seen there is CW3 which doesn't require Hegemony Rome
https://www.
moddb.com/mods/classical-world/downloads/cw3