Here's what I think you'll find to be true... but I haven't tried to do my gui yet in DarkGDK:
1: DarkGDK Takes over the WinMain entry Point - so many tool kits
that like to be in the WinMain might not be easily manipulated to work in DarkGDK.
2: In windowed Mode (Not full Screen Exclusive) I hear reports of people saying they can use normal windows, forms, and controls.
3: In FULL SCREEN exclusive mode - you should be able to get the DirectX GUI working like you would normal window gui stuff - though you won't be able to have the control you MIGHT need to keep DarkGDK managed gfx "underneath" the DairectX GUI. The BlueGui product I BELIEVE uses this DirectX means of getting GUI Gadgets....
4: IT's Much Easier to do with the DarkGDK.Net product - you can make DarkGDK.Net render to a "Bitmap" control (I think) inside a Normal Windows C# managed Form. e.g. System.Forms.etc
5: For Full Screen Exclusive Mode (especially in DarkGDK which is the C++ version) - it Sucks - and you will probably need to write your own.