I've never used the GoG version, but I believe it is the same as the regular One Whole Unit Blood. Go to set up your source ports via the menu in YANG, on the Blood tab, tick the "DOS One Whole Unit v1.21" checkbox and click the Locate button, find the Blood.exe file, this should be easy to find. Set your user maps directory too, probably just the directory Blood is in if you intend to use them at all anyway.
Now go to the DOSBox tab, tick the "I have DOSBox" checkbox and locate the DOSBox.exe in the same way you did Blood.exe, I reccomend downloading DOSBox seperately and using that (C:\Program Files\DOSBox-0.74\dosbox.exe) now for the config file, the default one runs Blood crappily in my opinion.
If you can find your DOSBOX.CONF file, save it somewhere and locate that as the configuration file. I would reccomend using a better one though or modifying the one you have. To get the file do this.
Go to the start menu and find the DOSBox folder in there, open "Edit Configuration" and look for Cycles, change this line to
CYCLES=MAX
This should remove any poor performance that would otherwise occur. Now save the file to a folder as a text file, probably the Blood folder as we know where that is and it won't get deleted, now you just need to select that as the DosBox config file in Yang and it should work.
Maybe I'll see you on-line? It'd be fun to kick your ass