Ten of them were coerced by a bored Ging Freecss.. Lol. Seriously speaking though, for me, it seems like Biscuit's theory is the closest one we got - to train Gon and for him to have fun.
Tbh I'm not sure if it was solely to train Gon, because I think it might of been to let any Nen user that tried to complete the game, making them stronger in the process. I mean Ging might be a shitty dad but he is kind of a cool guy... in that shitty dad sort of way.
I too think it was not only for Gon's sake, but I'm not sure it was made to help other Hunters get stronger. I think Ging's priorities were the following, in this order:
1- Train Gon while establishing a sort of remote contact with him through the game
2- Do something fun with a bunch of interesting people
3- Create something that could offer adventures to those strong enough to face them
4- Train other Nen users
Now we'll find out he's made it to prevent Pariston from conquering the world.
Yeah, Gon kinda stands out as the most probable reason GI was created. Ging even prepared the Magnetic Force and Accompany Cards for him once he clears the game... Ging had full control of the direction where the game ends.
What do you think?