Why would he need to "plead the 5th?"
Because he lied...? Was there no one in the car with him and he lied and said there was and now he's being forced to tell the truth?
OR
Was there really someone in the car with him and he's trying to "protect" them in some way from the big bad Chris Hughes?
OR
Was there someone in the car that he doesn't want anyone to know about because it's something else entirely/hinky?
OR
Is Gus just trying to play games?
we could really go out on a limb and say it was Skye in the car with him..... not.