A Gangster's methods are anything that makes money. In this regard he resembles the Merchant or the Rogue mission types. But a gangster isn't as much into solo operations, is more likely to build a large organization -- so long he is the only decision maker and the final checking account signatory, so long as he is the one on top.
If Earthlings gain admittance into the Galaxy and galactic culture, then they become protected and hands-off, which (for a Ganster) really increases the cost and risk of doing business. Right now, Earth is "wide-open," therefore cheap to operate in. If one of the agressive species or mission groups wins out, the same thing happens. In fact, if might be worse for a gangster is the Dracs or the Greys take over, than if Earthlings win out for themselves.
The Gangster, then, is concerned with keeping the status quo as it is right now, along with accumulating as much power and wealth and control as possible. He wants no one to win out, unless it himself. Of course, "winning" the Earth, for a gangster, might also mean complete defeat, for how would he then keep the Earth? He is unlikely to have the necessary military strength to defend his new possession. So, really, he wants things to remain as they are.
Given that, the most likely species or mission groups to harbor successful gangsters are the rogues or miscellaneous species, which sport many rugged individualists of varying ethical systems. Earthlings, Gizans, Antarcticans, Agharians, Greys and Dracs also make reasonably successful Gangsters. Bernarians, Pleiadians, and Andromedans make very poor Gangsters. Not but that they couldn't do it incredibly well if they were to so choose (especially the Andromedans), but the liklihood of actually choosing such an occupation is extremely low, and such a choice might even mar that individual's spiritual abilities.
In actual practice, the Gizans, Antarcticans, ED Humans and Dracs (or other Reptoids) are most likely to do the Gangster thing, and have the best advantage for such a mission.