The Gravestone is most effective when it appears at the top of an uptrend. It is the bearish counterpart of the Dragonfly Doji. The Gravestone doji's open, close and low are all at the same level at the bottom of the candle.
The Gravestone doji indicates that whilst there may have been some upward movement, the bears were strong and pushed the price right back down to the opening level. If you see the Gravestone doji at the top of an uptrend in which you are long, then take the profits quickly before the price reversal takes hold and is confirmed with a solid bearish candle to follow.