Runaway gaps happen in the middle of a directional move, often when a trend attracts new participants. The old name measuring gap comes from the folk rule that such a gap appears near the midpoint of the whole move, so the distance travelled so far can be projected forward.
That measuring rule has no serious evidential support and should be treated as a rough marker rather than a target. What is more defensible is the simpler observation that a market gapping in the direction of its trend is showing demand, not distributing.
Distinguishing a runaway gap from an exhaustion-gap in real time is essentially impossible, which is why traders using them tend to rely on trailing stops such as a chandelier-exit rather than on classification.
Related: breakaway-gap, exhaustion-gap, gap-fill, trend-following, chandelier-exit