One of my favorite cards from The Underway. Gang Sign is the "Criminal Clot". In slow-advance decks you keep it on hand until you see the corp advance an agenda in a server you can't get into. Drop it. Watch the Corp player's face break out in sweat. In fast advance decks you can drop it immediately to hamper their FA plays.
In any case, one way to look at it is that it is a card that adds an additional condition for scoring windows. There will be games where it does nothing. But there will also be games where it will close a scoring window where they would have been one otherwise.
Additionally, I really like the drama of the horrible situation it gets the corp in. Waiting won't do much to decrease the agenda count on HQ. So do you score one now and hope they miss the rest? Or do you wait for a Jackson and risk the agendas being stolen on an HQ run? Beautiful.
It synergies greatly with HQ Interface because you get to see more cards. It's great in Leela Patel: Trained Pragmatist since you can get deadly double-bounces off it. Could also work well in Iain Stirling: Retired Spook. I've seen people combine this with other on score effects like Turntable or Logos.
Overall, a great new card that we might see more of in the future. Certainly worth a try!