Anyways, I think that Leone takes this.
Gon is barely above baseline
8-B, scaling from a durability feat from Killua that wasn't casual.
Leone is stronger than Akame, who is stronger than Base Incursio Tatsumi who is much stronger than his base form, who stomps 4.98 ton monsters.
Honestly, Gon's only true advantage is AP. And even then, it's probably only via the Jajaken Rock.
"Oh but Versus, the Jajaken Rock is 8-A! Leone must be dead surely!"
No, in order to reach
8-A levels of energy, Gon needs at least a few seconds of windup, and even shouts the name of his move out loud. To an experienced combatant like Leone, who can sense hostile intent from others. This obvious attack would stick out like a sore thumb. Meaning Leone could easily evade it. And with each use of the Jajaken Rock, Gon's Nen capacity decreases, weakening. I recall that after 3 or so uses, he didn't have enough Nen to use it without risk to himself.
Knuckle_Bine managed to overcome Gon's power. He was able to interrupt the Jajaken Rock via striking Gon, rendering the attack useless. Leone is easily above Knuckle in smarts.
Leone with similar, if not equal stats would be able to wear Gon down, untill he is either incapacitated from over-exerting himself, or too beaten up by Leone to continue.
Leone takes experience and skill by a longshot, this one is a given.
She can also regenerate from damage she receives (Unless a limb is torn off or pulverized)
So, really Leone takes this Mid-Diff