I don't think Gon was trying to game the nen system. He does everything with maximum resolve. So when he stopped caring and decided to kill Pitou, he rashly and resolutely decided to fuck it all.
I also think any nen user can trade in all of his nen for "one last shot". But it's basically a suicide attack. If you run completely out of nen, you die. And if you don't use it all, you risk not killing the enemy off. Netero did this. Remember how old he got when he did the Buddha-laser thing? He just had a few minutes to live, at best, before setting off the rose.http://www.mangahere.co/manga/hunter...8/c297/15.html