How to get him
Gohan is one of those sweet, kind of shy and sort of dense boys with whom you have to just take the initiative. Subtle flattery will do the trick. Ask him to train you, or do something else to show you think he's smart/strong/hot/whatever.

Competition
Videl, who invented the above method. And since we're talking about that rarest of oddities, a trully GOOD boy, you'll have a REALLY hard time getting her off his brain. Not too mention the OTHER great love of his life. But then, nameks are asexual, so maybe Piccoro isn't really something to worry about.

He likes you if...
He asks you to play dress up with him. The only trouble is, he'll actually EXPECT you to go out in public wearing the stupid Power Ranger outfit.