It's not that complicated if you know the trick
Here's what you should do:
- download the game and extract it (best this would be on drive C:\games), so you will have a folder named C:\games\DGATE (and the game should be in this folder)
- after installing DOSBox (don't change any configurations) run it.
- type the following in DOSBox (after running it - if you changed any configurations this may not work):
mount d: c:\games (this will make DOSBox think everything you have in the folder C:\games - is actually your D: drive).
- now type CD DGATE
- DGATE
and the game should start (I didn't try the whole thing out right now, so if there's something wron just let me know)