Random Headcanon: Gon learns to play poker and gets really good at it. He challenges Killua to a game, even daring to make it strip poker. When he’s down to his boxers he remembers Killua knows a lot of games. (Killua wins)
I feel like Killua would be the type to count cards (plus he has a great poker face). Gon would never win. He’d probably ask Killua to help him get better (if only because he wants to beat Killua at strip poker).
Gon becomes a fantastic player but he can never beat Killua and it’s a little frustrating. But what he doesn’t know is that even though his poker face is perfect with everyone else with Killua his eyes shine with excitement whenever he thinks he’s about to beat him and it pretty much gives him away.