T.Ed works well with GDK, in my opinion anyway. It can export into formats that can be loaded into GDK(3 formats I think)..
You can either Export the whole terrain as a big .X model(or i think it supports .DBO aswell) to load, with a supertexture, which is one big texture that is draped over the terrain.
or you can export your terrains as heightmaps, with a hole bunch of options for alpha and environment maps, and load it into GDK as an Advanced Terrain object(or with your own mesh if you choose)
or you can export it into T.Ed's native format and load that directly into GDK. It saves the world as a bunch of small .X object tiles, and saves all their position, texture etc info into the .ted file, which you can parse thru with cpp to be able to load. There are a couple of T.Ed loaders for GDK to load .ted files aswell that people have made, you should be able to find them by searching these forums.
For the price of the product, I think its pretty damn good.. although Im not sure if its being updated anymore, the latest version has very few bugs that I have noticed.
If it ain't broke.... DONT FIX IT !!!