My question is about the term "the thug on the corner". It doesn't seem to convey just its literal meaning. The context reads "Almost all field research sites have a gatekeeper. The gatekeeper can be the thug on the corner... or the manager of a cafeteria. It is good practice to ask permission of gatekeepers before your proceed."
As I understand it, PMsmart, you have two examples of a 'gatekeeper'. One is the manager of a cafeteria, whose permission you should seek before proceeding ... (to do field research? ... in his cafeteria?). The other is the thug on the corner, whose permission you should seek before proceeding ... (to do field research? ... in his street?).
In each case, the 'gatekeeper' may object if you do your research without asking permission; but (as sdg says) nothing in what you've given us determines that the thug is a drug gang member — he may just resent an intruder's lack of respect in sniffing around in his street/area without asking a local if it's OK to do that.
"The thug on the corner" has its literal meaning just as much as "the manager of a cafeteria" does. The cafeteria manager may also be a serial killer, or he may be stealing from the petty cash, just as the 'guardian of the street corner' may also be a drug gang member, or a pimp, or just someone who may steal your car wheels if you don't ask permission nicely — but those are separate issues.