I couldn't care less if I met him or not. I wouldn't ask him anything.
I would say oi make me in the next gantz and im cool and have plot armor.
I would ask him to release gantz chapters every day of the week.
Assuming I was selected to participate in Gantz prior to meeting him, I would seek advice regarding Gantz.
I would ask what he thinks of when he draws women with big breasts...
Then I would ask if he could draw more naked alien chicks.
I would say, approve the petition of weekly gantz release.
Are ya with me on this :P?
Probably break down in tears and tell him I want to do what he does for a living and that he is living the dream. :|
I'd want to ask him like dozens of questions about gantz that I want clarified, but my Japanese is nowhere near that good. I'd probably just ask for his autograph and a picture or something.
I would say for him to stop being lazy and release the episodes in a weekly basis again. He stopped doing it because of Meteru no Kimochi (which was a 28-chapter failure) and never published Gantz weekly again.
Ask him if there will be a retcon of the anime.
I DEMAND MORE NISHI
Since I don't know japanese I would gesture a coke bottle shape with my hands follow by a thumbs up and laugh. Hoping he won't call the police. I would ask someone to translate "When will he plan to release 2nd data book and wtf is gantz", knowing full well he will say I will eventually learn in a future chapter.