Alright. I've looked into Big Boss's profile, so I'll give my input on the outcome of the fight.
Gon should take the lead in combat skill, as he is shown to be as cunning as Big Boss, and a strategist, as said earlier when he managed to use someone's psychological response to an incoming object to his advantage, using his surroundings, etc. The advantage Gon has is by no means great or anything, but still, it's present nonetheless.
Big Boss absolutely dominates in versatility considering his many weapons, and ways to stun or attack Gon, on top of Accelerated Development to quickly compensate for the skill gap in the midst of combat. Surely he won't surpass Gon outright, as Gon also has Accelerated Development, and can keep up with people more skilled with him as he did with Knuckle, so the gap is not going to be inversed, but eventually, both will be comparable in skill.
Gon, however, possesses Reactive Power Level, meaning he will progressively get faster, stronger, and more skilled, while Big Boss only gets skill out of his hax, therefore Gon will get a continuous advantage in a physical sense. Furthermore, Big Boss' guns wouldn't be effective,
as bladed/piercing objects can simply be blocked by a powerful Nen, and since Gon even at baseline is twice as strong, and can go up to 100 tons in Durability, it's likely his darts/bullets will simply bounce off Gon's Nen and won't be able to put him to sleep at all.
Considering his Invisibility and Sound Manipulation, Gon might have trouble finding him, but thanks to Gyo which can see invisible Nen, he might be able to see him. Even if he can't, this can easily be countered by Gon's skill, as he will easily figure out that he only needs to take a piece of the scenario and make an omnidirectional attack in order to find the invisible opponent (much like it was discovered by Youpi during the Chimera Ant arc), thus, overcoming said ability as well.
Gon's advantage, that being nearly 5x with Ryu, will allow him to effectively decimate Big Boss on the first connected hit, likely breaking whatever he hits, and by considering Gon's extensive knowledge on HxH's version of Kung Fu, Gon should use this monstrous advantage to connect a hit in a specific spot in order to knock him out instantly. (7.5x gap is only for a hit on the chest, a 5x gap on the chin would send the dude to the shadow realm instantly)
About sleeping gases...
Gon can hold his breath for about 10 minutes, and literally did that to escape sleeping gas, he has experience with such.
Basically, one hit to the jaw, or a well connected hit with martial arts knowledge, and the opponent is out.
