When talking about the individuals who work for contractors on behalf of Northern, who have very little training and very poor social skills in handling customers, many people would consider it an apt nickname.Yup, every day.
So why not insult the fare evaders rather than the staff employed to try and stop them.
To refer to them as Gestapo is not really funny is it?
You may think they stop fare evaders but they've even been known to stop bona-fide rail staff (which they aren't) from accessing the trains they're supposed to be driving, to give just one example of their behaviour.
If you had been spoken to them in the way that many law-abiding people have been, I suspect you would then realise why many people are unhappy with them.