In my experience, trading the assumption that a gap will be filled, or the opposite, are both profitable trading strategies (certainly for earnings gaps). The key is to consistently trade one way, either long or short, to profit. Personally I trade NYSE stocks long (assuming a gap won't be filled), NASDAQ short (assuming gap will be filled). Both work well, however there are times where there are a string of losses that have to be traded through. Consistency of approach - the market never stays the same and the good times do return (often when you least expect them to).