A gap is a price range in which no shares (stocks) or contracts (futures and options) are transacted.
A gap up occurs when the low of day 2 is higher than the high of day 1
A runaway gap is found in the middle of a trend, usually when the price has already started a strong move.
It is a healthy signal as the trend will continue as it indicates even raising, interest in the stock.
In an uptrend with a runway gap up, Bulkowski (2005) states that on average, the gap's center occurs 43% of the way from the beginning of the uptrend to the end of the uptrend
similarly, a runway gap down occurs on average 57% of the way from the beginning of the downtrend to the end of the downtrend.