Honestly, I can see a team selecting Gagner early this draft based more upon his recent play in the playoffs than his regular season, and of course those NHL 'connections' which some prefer to call bloodlines, does not hurt him.
He is playing somewhat hot & cold in these playoffs, but when he's hot, he is a standout on the ice especially in crucial PP situations for London.
Of course he benefits greatly from his father being one of his coaches & being handed more icetime than almost any junior player could handle, but so does Patrick Kane. Gagner lives & sleeps in his family home so he benefits from that more than most juniors including Kane.
But when it comes to playing the game on the ice, all season Gagner has demonstrated an ability to produce offence, while maturing along the way.
I honestly don't expect to see him "drop" out of the top 5.