A gang-banger is a violent gang member, say like a Blood or a Crip. You don’t want to make eye contact with them. Neither do you want to make eye contact with a Hell’s Angel. They will probably take it as a sign of disrespect and as a challenge.
Dealers and pit bosses can spot a counter in minutes, but they don’t care about clumsy beginners. If you’re a good counter, and there are a lot of them, they will often expel you from the casino, and the face recognition software, or at least video surveillance record will be part of a report that is, as far as I know, available at least to all the other bigger casinos. You go on a “black list” and have to go way out of town to play cards. If you get on a list, you may not even be allowed to ever re-enter the premises, even to eat or see a show.
This is what I’ve been told by security heads and also by acquaintances who were expert counters. Some of them still play, but they have some tricks to avoid expulsion. I don’t understand how those tricks work (and couldn’t reveal them anyway).