Gaps are rubbish. All it means for a "gap to be filled" is for the price to pass the same area again. Of course they get filled.
Having said this, there are obviously some traders who take a lot of notice of gaps. For instance, the CSL dump on Thur was bought at exactly the bottom of the gap and if you were aware that traders were likely to do this, you could have joined in for a handy profit.
