You can use all kind of text commands on your picture. Copy, paste, undo, edit all units. There is no hidden functionality. You can see all object properties. No more wonder how someone did this effect.
Gilt is postscript.
There is no need to invent a new picture description language because there already is one. This means you can import all postscript pictures and modify them. In fact you can do a lot more than that. You can write your own programs. This seams very complicated and you are right.