What a gap is
A gap is the space between the prior regular-session close and a later print, often the premarket price. It is a percentage of that prior close.
You need both prices. One price alone is not a gap.
Above the prior close
The premarket print sits higher than yesterday's close. The size is the difference divided by that close. A wide spread can make the first print look larger than the next one.
Below the prior close
The premarket print sits lower than yesterday's close. Same arithmetic. The cash open can give the move back, or it can extend it. The open print is a new price.
What it is not
A large gap is a description of two prices. It is not a reason, by itself, to do anything. News, a filing, or a halt may sit beside it. Read those pages before you treat the gap as the whole story.
General market news. Not investment advice. The author may hold the Runner Flag name and does not trade it before the 9:30 ET open on the publication day.