The actual max reslolution is 1024，when it is forced scaling in 2k screen, the graphic become shabby. How to cancel the forced scaling? I've tried the nvidia setting but it doesn't work.

Set it in %AppData%\GOG\DungeonKeeper2\config.ini, add a line:
ScaleMode=2

I think 0 was stretch & fill, 1 keep aspect ratio, 2 center without scaling

Some documentation by timeslip:
https://www.gog.com/forum/dungeon_keeper_series/dk2_gog_configini_extra_settings/post2