Stronghold Crusader HD is a package of 2 games: the original Stronghold Crusader and Stronghold Crusader Extreme.  Each game uses a separate exe file.

In GOG Galaxy the "Play" button launches Stronghold Crusader Extreme.  In order to play the original Stronghold Crusader please select the game in GOG GALAXY and click the customization button in the top bar → Additional executables → Stronghold Crusader HD