I'm only seeing the base game of Assassin's Creed Syndicate on Steam, and no Gold Edition. However, I can see the Gold Edition available on both Epic Games and Ubisoft Store.

I'm unsure why Steam is only offering the base game. Is it possible that the base game on Steam actually includes the Gold Edition content? Please see the attached link for a visual reference.

https://ibb.co/RhSzPb0