The limitation is in the XNA framework. There is a guy Michael Russell that have made a custom launcher/mod for Terraria that enables higher resolutions (http://romsteady.blogspot.se/2011/05/terraria-trainerresolution-launcher.html
), Terraria is also XNA framework so it should be possible. Try sending a email to that guy and see if he is willing to mod Gnomoria aswell.
Another possible way of making it work would be to use the OpenGL version (Linux version) of the game, it does work in 4K resolution. Maybe there is a way of porting the OpenGL version to Windows? I mean u can port stuff to Linux so why not the other way around?
The Linux version acctualy have a exe file in its folder and i have tried to get it to run on windows using mono (seems to be what linux uses to get the game to run), i was not sucessfull in this attempt but others may be better at that stuff that me.