The place I always go to when I have that same question about any game for PC is
https://isthereanydeal.com/
As for this game, according to the site it was in the October Monthly Humble Bundle, as well as a previous Humble Bundle, both of which are no longer for sale...
https://isthereanydeal.com/game/americantrucksimulator/info/

It doesn't just track bundles (for this game you'll find the past bundle listings after the sales listings on the link above). It also shows current sales on Steam as well as 3rd party sites that sell codes for Steam games, historical low prices if you wanna see if a game on sale has ever sold for less, and recently (at least I think it's a recent update) they have added a filter that lets you search for games that activate on UPlay (Ubisoft), Origin (EA), and DRM free games (I assume listings for GOG.com would be found using this filter).