One thing is for sure -- if it's a fact that could influence the sale or the transaction (aka material fact), it must be disclosed. Just wording it might be tricky. Check with your agent.
As a homeowner you must disclose what you know to be a fact. If you are having some questions about disclosing you want to consider getting some professional price.
Could it just be teenages hanging out? What is prompting you to suspect gang activities?