bribes DD is simple and really good, and its not really written in GUI (it consists of many jass scripts), but its made for GUI usage
so for studying its exactly what youre searching for
but if you want to study a dd system that uses almost every vJass utility (because you were asking for vJass and not jass), then go for nestharus damage system:
damage event, advanced damage event, damage modifcation effect (study them in this order)