The trouble tends to come with older stuff maybe using odd libraries, odd apis, possibly still using assembly code, being made for vastly different graphics standards and more things along similar lines.
Depending upon the project then one of the earlier things seen in most open source recreation efforts tends to be bringing it into line with modern standards.
A quick scan of the code says actually a lot of it is fairly plain as it is which is nice, and it works on a few different operating systems as well (though I am not sure if it relies on wine a likes to provide a layer).
is the PC Linux build requirements in addition to the opengothic library itself. I don't know all of those and what they have inside them to act as compatibility layers, and what equivalents the Switch might or might not have.
This then leaves how easy it is to go from the assumptions of the game for PC to what the Switch has in hardware as some of the audio and graphics stuff did seem to be more on the directX and abstract side ( https://github.com/Try/OpenGothic/blob/master/game/graphics/renderer.cpp as well as the stuff in the shaders folder) rather than mimicking what is in the Switch hardware, though abstract stuff is still somewhat generic rather than anything too outside the box (I suppose it was a 2003 game rather than before the directX takeover or geared for consoles of the earlier eras, and before shader coding became really hot stuff).
This also looks to be a somewhat incomplete version at present
Core gameplay is done, you can complete the first chapter, as well as all addon content for any guild.