What's GPB? Anyway, once you've got a grammar figured out, you need to write a parser for it. You need to parse the program text into some sort of data structure. For instance, in LISP, that structure is just a pair of pointers (and all the lists and trees you can make out of that). For you it would involve a list of functions and a list of statements for each function and etc. Finally just write a function to execute that structure.
It seems that GPB is going to take care of the parser for you (with the Engine bit, whatever it outputs). Although I'm not sure why use anything at all. Parsing isn't that hard, especially if your grammar is simple enough for a recursive descent parser. The task is generally automated, because all parsers are pretty much the same (same methods, only different grammars), but the whole point of making toy projects is to learn, so you might as well do it yourself...
bison already creates the parser code for you, so if you think your grammar will grow more complex with time, I would move to it.
Else writing an object oriented recursive parser is not that much to do.