As far as the command-set goes, its pretty much exactly the same except the commands are all one word and have 'db' in front of them.
Example:
Make Object Sphere 1, 50
becomes:
dbMakeObjectSphere( 1, 50 );
The main difference is that you'll be using c++ syntax for everything else. Some of the operators and other utilities that were available in DBPro aren't available. For example, in DBPro I believe you can combine all types of variables and it will do the work for you. For example, ( Text 10, 10, "Health: " + Health ). However, in C++ you can't just add numbers to strings and have it work out. It's not very hard to accomplish the same tasks, but it's different, and will take some getting used to.
I'd follow some basic C++ tutorials before jumping into DarkGDK. There are some nice tutorials at this site: www.cplusplus.com
Good luck