The issue here is that the G-league games are practically meaningless. They're basically scrimmages. The teams' personnel are a revolving door, guys are motivated to develop their games without getting hurt, and the games don't matter. If you want to see what Brantley looks like in an NBA situation, you eventually have to put him in an NBA situation. Give him 8 minutes to see if he can defend, rebound, set screens and knock down his shots in a real game.
Of course he's going to cruise if he's playing 30 minutes per night in a glorified summer league. He's putting up 20/7.5/4 and isn't going to prove much more in that setting.
He has the talent to be the next Paul Millsap. Yes, he needs to play harder, but I don't see why he's going to go full throttle in the G-league.