The most common way to play for free is through limited-time events on major storefronts.
: Steam and GOG occasionally host "Free Weekends" for both the original game and its sequel, Battlefleet Gothic: Armada 2 . These allow you to play the full version of the game for a 3-day stretch. Battlefleet Gothic: Armada Free Download
If you are looking for the original tabletop experience that inspired the video game, much of it is available legally for free due to its status as a "Specialist Game" no longer actively sold by Games Workshop. The most common way to play for free