This is a very simple gap indicator derived by comparing the previous opening and next closing price.
Gap down if opening price is lesser than previous closed price.
Gap up if opening is more than previous closed price.
This gap detector use to evaluate the continuation or reversal of next candle and needed to be implement with other indicator such as Moving average or Boilinger band to read the possible direction of trend. Basic knowledge of candlestick pattern may help to predict the direction of next candlestick base on gap up or down of current price.
Alert can be setup using this gap detector accordingly to the time frame of chart.
Multiple time frame is recommended