In candlestick charting, "gaps" are called "windows". There are rising (bullish) and falling (bearish) windows and the rule is to trade in the direction of the window because they are considered continuation signals (buy the dips on rising windows, sell the the bounces on falling windows).
Price corrections tend to stop at the windows (a rising window will act as support on pullbacks and a falling window as resistance on bounces), which often results in the window being "closed", thus the belief that all/most gaps are filled. If/when the window is "closed" and price continues beyond it, the sup/res is overcome and the current trend dictated by that window is finished.
A bit over-simplified, but hope this helps. See Nison's books on candlestick charting for more info.
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