Could Alluka completely heal Gon?
We have not seen Gon used his Nen since he's been healed, but he does look perfectly healthy. I think the answer lies in the way Kirua phrased his wish:



This picture was taken from "mangapanda". I have the books (in French) and it says "Return Gon to the way he was". Also, page 7 of chapter 323, Illumi states that Kirua will most likely ask "alluka to make Gon as he was before".

Therefore, I think Nanika brought back Gon to his state (physical, Nen...) before his fight with Pitou, as if it never happened. So, any restriction or contract Gon imposed on himself in order to defeat Pitou has been canceled. 

Again this has not been confirmed anywhere in the manga yet.