Havent seen that particular Ross Kemp on Gangs but I have seen a few "Real Football Factories" on Bravo and yes they do tend to be a nasty bunch.
The odd thing is when they get older they do tend to calm down, you can see quite a few members of the 657 Crew back in the 80's going to Fratton Park kids in tow, and while you wouldnt want to get on their bad side they can be quite nice guys.
Jen