Gon has slept with multiple women as you know it has been mentioned in the Anime. So Gon knows multiple languages including Urdu. He thought he will be a cheeky guy and try to use a curse word in a language Killua won't understand. But coincidently Killua knows what it means. Because he once had an assassination mission on which he had to learn Hindi. The word "Ma choda" has the same meaning in Urdu and Hindi. Gon was so shocked that he gave no reaction to Killuas's reply and just pretended nothing happened.
This is a sad reality for kids nowadays. It's a rumor that Yoshihiro Togashi the author of HxH tried to reference Pakistani kids because they know curse words better than Adults and Jujutsu Kaisen characters.
This is a satire article just in case you are dumb enough to believe it.