RE:Not a Technical Analysis expert
The short answer is no.
Statistical probability is yes but it is not MUST and sometimes gapfills can take years to occur.
Often the bottom of gaps will become strong support during a resting phase and during retracing movement in a down market. I look at, at least, three things to judge gaps,gapfills and directional movement,
1) The upper Bollinger band at 2 standard deviations
2) The lower Bollinger band at 2 standard deviations
Price movement outside of these two markers are outside of statistical probability. If price is above the upper line you should at least be thinking about trimming your position.
If the price is below the lower line you should at least be thinking about adding to your position.
3) OBV (On balance volume) the best way to tell if the volume trading is people selling or buying and also gives indications of how much emotion is built into the prive movement.