I contacted the author of this question before providing a solution so that I could get a clear definition of the "gap" they wanted to measure.
Response:
In this case it would be the difference between the price at the open and closing price of the most recent candle.
This means the solution would print one value for the current session and that value would not change as the current session unfolds. In that case, we go with the very simplest solution possible. Set the watchlist column to a daily time fame and add the following line of code to a new custom column:
plot data = open - close[1];
def dailyPercentChange = open - close;
And apply this to a daily time frame as described in that previous question.