A lot of articles in wikipedia have links on any random word, which in my opinion is not good use of links.
Example of poor use of linking: "A Tree is a form of plant life on Earth
. They get their energy from the Sun
Links should mostly be to articles relevant (not tangential) to the one being read. Also strange technical terms that the reader may not be familiar with. So an article on A-star can link to 'Search Algorithms'. It can link to 'Priorty Queue'.
Exceptions to the 'not tangential' rule may be people's names (for their biographies), or articles about specific pieces of software, if one happens to be mentioned.
But that's only my opinion. I've never been fond of wikis though.
As for links internal within gpwiki vs external to wikipedia. I would say either are okay. The best choice is which article is most useful. But if it's a close call, I'd prefer to see links to gpwiki articles.
As for quantity. I think somewhere between an average of one link per sentence and one link per paragraph is optimal.