To get Gunn's line, you need to have more points with him after the fire than with Ron. The most effective way is to go and defeat him in the arena, you can also get points if you cooperate and go look for a mechanic with him.
Also, to get a completely nude scene with him. You need to score maximum points with him. Choose shortway, in the moment by car, but this may not please the crocodile, because he is afraid of trouble.