It has been a while but I will try to answer with respect to the discussions held during that period.
First I would like to point a golden rule: Correlation doesn't imply causation. If Adult Gon and Illumi both have long hair it is NOT necessary they have it for the same reason.
Illumi has long hair because his character design was meant to be that way. He was meant to be introduced as a very creepy, gender-fluid character who could easily dominate the "imperturbable" Killua. The hair design gave author to do a lot of things with Illumi.
You've arrived at the reason for Gon's hair being long correctly, I believe. During Gon's transformation into what Killua considers "How he would appear after years, if not decades, of training", we also notice that his hair has grown absurdly long. This implies how much of time he would need to spend training to improve his Nen Ability.
To end with this is an old image with Illumi's character design (Hair). I'm not sure what does the Japanese text says.
Please Note, I know this answer is very opinionated without author notes to back it up. It is drawn from discussions during the original manga release which I have not tracked. In this case its the best answer to the question I could think of.
Feel free to suggest ways to improve or any flaws in the train of thought, I might have missed. I also haven't followed HxH in a long time so any new information is very helpful. Cheers!