Casual:
* Yo, what's up?
* Sup, man?
* Hey, how's it hangin'?
* What's shakin', baker? (slightly playful)
More serious:
* Hey, you. (Short and direct)
* Listen up, pal. (Indicates you have something to say)
* We need to talk. (Implies business)
Menacing:
* You lookin' for trouble?
* Don't make me repeat myself.
* You know what happens to snitches, right?
Remember, these are just examples, and the best way to say "hello" in a "gangster" style depends on your specific situation and the tone you want to convey.
It's also important to note that using this language can be offensive and perpetuate stereotypes. If you're using it for creative purposes, be mindful of the impact your words might have.