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It was my understanding that Gaps are an area where there was little to no liquidity or back and forth trading (I know that term does not make much sense, but I'm not an expert so bare with me). There is hardly any resistance in these areas so when price goes into the area there is nothing to stop it continuing to fill that Gap. All this seems to offer is a higher probability for closing the Gap and, as seen by history, not a certainty.Ignored
The gap you are talking about is a momentum driven gap which can exist any time of day. Those exist because of the order imbalance in that particular moment. Simply put, someone is demanding more through his market order than is offered through market and limit orders combined. Those gaps are best seen during high impact news events. On a smaller scale, mini gaps happen very often.
However, that doesn't mean those intraday gaps must be filled. Just because there wasn't orders existent during the gap creation doesn't mean there won't be orders when price reach back "into the gap". As Piptrapper put it, they tend to fill because of the self-fulfilling prophecy