I strongly encourage read & review John Murphy Technical Analysis of the Financial Markets. It's a freakin' bible IMO.
You should review ALL of chapter 4; with especial attention to "Reversal Days" (p. 90), Gann & Fibo Fan lines; Speedline retracements; and the Three Types of Price Gaps including their characteristics:
It is here written; "Breakaway gaps more often than not are not filled; prices may return to the upper end of gap, and may even close a portion of the gap, but some portion of the gap is often left unfilled."
NB: 14 October gapped up 40 pips, slightly pulled back and then went parabolic.
Measured, aka runaway gaps, sometimes go unfilled, but often act as support and remain open, whereas "after breakaway and runaway gaps have been identified, the analyst should expect the exhaustion gap... which will quickly fade and prices turn lower within days... it is a dead giveaway... the exhaustion gap has very bearish implications."