It may be a good idea for public safety in the community if gang membership and criminal activity is reported to the police. Someone who is posting involvement in criminal activity online may be fabricating it, or it may be real. If you report it then the police can decide how to handle it. If you are concerned about your own safety you might consider reporting it anonymously, perhaps printing the information out and mailing it with a note to police, explaining your concerns.
*Not legal advice, and you are not my client.
*NOT LEGAL ADVICE. YOU HAVE NOT PAID ME A RETAINER. WE DO NOT HAVE AN ATTORNEY CLIENT RELATIONSHIP. This is general information for educational purposes only. You should always hire a lawyer and reveal all the unique facts to get the best answer for your unique situation. Answering this question on a public forum does not create an attorney-client relationship.
Will the police taking it seriously? Probably not. Unless there is some specifics as to crimes the person has committed or is about to commit, I can't see them doing anything at all. One thing about the internet is there is a vast chasm between what people post on facebook and other social media sites verses what is true in reality. I wouldn't get yourself worked up over this as if the person is actually in a gang and talking about it like they are on the internet, it won't be long before they post some self-incriminating picture.