Hmm. Well, first off, the GDK of DarkGDK stands for "Game Development Kit"
Now there are lots of reasons why you would want to start of with DarkBASIC Pro, firstly, its MUCH easier than its C++ counterpart, and easier to "jump right in" and start making games in, so basically the learning curve isn't as steep.
As for GDK, you HAVE to have some C++ knowledge to get a good understanding on how it works straight away. While I mostly use GDK over DBPro now, its because as you said, its faster, I come from more of a C++ backfround, and myself personally like a nice tidy code, it gets me back in the habit when I program in GDK.
Now if you are new to programming, I recommend you go for DarkBASIC Pro, create some simple stuff and see how it goes.
Or if you wanna jump right into GDK, then it is recommended that you learn the layout of C++, how it functions etc.
This is a nice little site, with free tutorials, it should get you started right away
http://www.cplusplus.com/doc/tutorial/
One thing is for certain though. After some middle class knowledge in DarkBASIC Pro, DarkGDK WILL come to you that much more simpler.
I also recommend downloading the DarkBASIC Pro trial, play around with that, see how it goes, try before you buy, its a good tactic
Happy programming!
Eidos!
