Simple answer: the output of gvpr is the Graphviz / dot language. The grid lines are a result of adding new nodes around the periphery of the graph and then adding (straight) edges connecting the nodes.
More complete answer: gvpr is a complete language and could probably be used to write a (very simple) compiler. (Not on my TODO list). Here is a lint program written in GVPR. No dot output, just text.
For an example of the breadth of gvpr, here is a radial layout engine written in gvpr. Legal dot input and legal dot output.
