I think DarkBasic Pro and FPSC are TGC's "flagship" products.
DarkGDK has been around in one form or another for more than 5 years already, it was originally a commercial product, similar to DarkBasic Pro, called DarkSDK.
I cant say 100%, as you never know what will happen in the future, but from all the signs and what has happened in the past, I would say that they are here to stay - at least into the medium term future as you asked.
(For some hint that they are still looking to the future, have a look at some of their other products and where they are heading with them, directx10 support etc - plus they are always adding new products to their lineup, tools and whatnot)
If it ain't broke.... DONT FIX IT !!!