You can either export them as .DBO format object and load them directly in GDK with dbLoadObject() or you can save the file as a .3DW (the native 3DWStudio format) and load it into GDK yourself.. The native 3DWStudio is basically a text format, listing what is where in your level, I beieve a couple of people have written loaders for 3dWStudio to load 3DW files in both DarkBasic and DarkGDK...
Have a search through the forums for "3DWStudio Loader for DarkGDK" ... (I think Pixel Perfect wrote the GDK one, and LIT originally wrote the DarkBasic loader)
If it ain't broke.... DONT FIX IT !!!