You could prove gender by telling everyone to change their avatars to parts of their anatomy. Anyone who posts male parts are guys. Anyone who posts female parts are also guys.
In all seriousness, it's a nice idea, but I agree that the anonymity of the Internet is problematic for organizing such an event. That is, unless you don't really care who claims to be what, in which case, I say go for it.
Edit: I think you may have missed the subtlety of the joke, Martyrose :-)
I like the idea, but Dan is right. You can't proof your gender. And good luck with finding enough girls for the tournament.
Edit/Responce:And traviter how can you proof that people are posting their own parts? Also your parts don't have to influence your gender, but your biological-sex does. Ask Lawlz if you don't belive me.
This reminds me of when I used to pretend to be a girl.
About 15 years ago I played chess on yahoo. If I won, my rating would go up. I am a bit competitive and I liked my rating to go up. But I was bad and would use any trick to make it go up. One of those tricks was pretending to be a girl.
There were chat boxes in the game. The usual initial question was age?sex?location? Being 19 and a girl was an advantage to being 33 and a bloke. The internet in those days was not as pleasant as it is now. Most chess players are blokes, if you were a girl about 25% of the time would result in degrading, disgusting, unpleasant chat from the bloke on the other end. I would simply minimise the chat box so I could not see it. I would be concentrating on the game, the bloke on the other end would be concentrating on something else. I would win more times than I should. Result!
There was one game when I pretended to be a 33 lady. That was my age so it would be easier for me to pretend to be my age rather than someone much younger. In this particular game it was an advantage as the bloke was in his 40s. He was from Egypt and the chat was very pleasant. We exchanged a lot of pleasant chat, I told him I was a policewoman, liked cats and photography. He invited me out to holiday in Egypt. "That would be lovely", I said.
Later he asked me to sleep with him. I remember laying it on thick and saying, "What kind of girl do you think I am?"
Things went quiet.
Unfortunately, he was a very good player. I was getting thrashed.
Then he obviously looked in my profile. "Are you a bloke?" I could never work out how to change my profile from a bloke. "Yes", I said. "Are you gay?" "No." "Are you internet police?" "Yes."
Then for some odd reason he resigned the game he was winning and went offline. Game won. Result!
A tournament of girls v boys would be pointless as you get some wierdos who pretend to be girls.