When Gon transformed against Pitou, his Nen increased a lot. Why didn't we see any reaction of people feeling it like The King, the other Royal Guards, or Netero (if they were still alive) ? In the Election Arc, we saw everybody feeling Alluka's power, as it was very big. I could say the same with the King's birth, because he was born with Nen.
There are multiple factors to it:
1) Distance: Gon and Pitou were a substantial distance away from the main combat areas.
2) The varying use of In and En by the users. During combat scenario this is very crucial vs the election which was a more friendly environment. Most of them were in their own active battles concentrating on either sneaking or fighting.
3) I believe they tend to pick up the largest active source of nen. When the king was born, Pitou was on the look out and used her nen (En) to channel it out in a defensive grid surrounding the nest. Everyone was so afraid to get near the nest that they wouldn't have been close enough to feel his nen in his passive state. Pitou could also have potentially masked or partially masked Gon's nen in their fight since she was fleeing away from him towards the others.
4) Gon in an enhancer, they fight by concentrating their nen into their own bodies, along with everything else on the list this would contribute to why they couldn't feel it since he tends to concentrate his nen more inwards.
And I maybe be wrong about this last one (been a while) but I thought Killua who was closest did feel the difference.
Gon never used Nen against Pitou. He uses On (怨, Malice) http://hunterxhunter.wikia.com/wiki/On, which is fueled by hate and vengeance.
'On', used in the 'Last Mission' movie, does not alert other Nen users when emitted in great volume.
Gon felt used and lied to by Pitou since she revealed that Kite was dead and unable to be revived. He summoned this malice and hate to mature and kill Pitou.
Gon's aura after and during this transformation is a combination of yellowish/orange and black, exactly the same as his eyes when he fought Jed in the 'Last Mission'.