look in the log files at \..\Valve\Steam\SteamApps\common\Life is Feudal Your Own\logs\
the logs are txt format so .log file can be opened with notepad, when open in the first page you will see:
<createCanvas> Attempting to create GFX device:
GPU name hereyou will probably find in the log it will be reporting the correct GPU, its just that something is amiss in showing it in the in-game graphics device drop-down menu, so it defaults to displaying "ATI Radeon HD 5700 Series (D3D9)" even though its not using that.
Mantis - 0001301: Nvidia GTX 780 shows up as ATI Radion HD 5700