At first glance, forcing a single-player-focused game (the campaign is its main draw) to require Steam seems excessive. However, from a developer and publisher perspective, there are logical reasons:
Unlike the early days of PC gaming where a disc and a CD key were all you needed, Gunslinger represents the modern standard of digital rights management (DRM) and service integration. Here is a detailed look at why Steam is not just an option, but a mandatory requirement to play this particular Western adventure.
Steam acts as the digital rights management (DRM) layer, verifying your license every time you launch the executable . 2. Platform Verification & Steam Deck Support