Personally, I feel you should answer "yes" if you were once GCD. The reason is simple, because if USCIS finds "yes" and you say "no", you can see the consequence. On the other hand, many ones said "yes" before got approved. That's another reason.
However, it totally up to individuals, your choice and your decision.
I don't agree to fight for this, or even smear someone who just disagree with the other one. What's the point? Anyone can do whatever they like, no one forces them. It is not "win" or "lose" game, because even someones "win", what they would gain?
Therefore, this argument should stop.