I think you're right. I believe it has much to do with the attire of the characters. When any such characters are doing dirt dressed in suits it is believed they are gangsters. Really though, gangster is a term more directed at mob figures which Heat does not really address.
Anyone who watched chicano gangster movies ever remember "Boulevard Nights" lol? Waaaay before American Me and Bound By Honor..... That movie had clean ass lowriders, especially that old school Monte Carlo.