Was just looking to buy the Gathering Storm expansion yesterday, glad I held off. Price drop of 40% at Green Man Gaming, until 1AM AEST tonight.
Was $54.95, now $32.97. https://www.greenmangaming.com/games/sid-meiers-civilization…
Note this is just the latest expansion, you need the base game to play. If you haven't already got the base game, GMG also has the Gold edition available for $34.93 (as per previous deal) which includes all previous bonus civilizations and the previous expansion Rise and Fall - but not the Gathering Storm expansion, obviously.
https://www.greenmangaming.com/games/sid-meiers-civilization…
Snapped it up, thank OP. Also, it works for mac even though it seems to indicate it's only for PC.