Not true, gaps do not always fill. Usually, yes.
Post# of 30027
Gaps: Important Bull Market Results
Percentage of gaps closing within a week, upward breakouts:
Area gaps: 89%
Breakaway gaps: 2%
Continuation gaps: 4%
Exhaustion gaps: 61%
Additional information on gap closings.
Area, common or pattern gaps 3 days for both breakout directions
Breakaway gaps 136 days for upward breakouts, 168 days for downward breakouts.
Continuation, measuring, or runaway gaps 98 days for upward breakouts, 77 days for downward breakouts.
Exhaustion gaps 9 days for upward breakouts, 14 days for downward breakouts.
Hope that helps.