I can't give you an answer that will apply in all cases. You're going to have to review charts yourself and make your own choices, but twenty or thirty ought to be enough to enable you to work out some probabilities.
First, you have to look at the size of the gap. According to Frederickson, 71% of gaps of 1.5% or less fill. Once they get wider than that, the fill rate is cut in half. So you may decide that you're not going to play gaps that are greater than 1.5% due to the probabilities.
Then, look at these gaps that fill and note how fast they fill. Do they all fill by 1000?
Then, note how often the price fills the gap then fades. Note how often the price fills the gap, retraces, then resumes its move, creating a trend.
Then, make some choices as to how vulnerable you want to be. Using a long as an example, do you want to let price come all the way back to 5pts below your entry price? Or do you want to set a much tighter stop in the event of a fade or a retracement? If there's a fade, are you going to play it? How? If there's a retracement, are you going to play that? How?
Filling a gap is not creating a trend. Filling a gap is almost autonomic, like a sneeze or a hiccup. A trend requires a bit more conscious effort, which is demonstrated when the gap is filled, price retraces, then resumes the original direction rather than retreating back to a neutral position.
You may decide, however, that you just can't or don't want to screw around with any of this so early in the morning. Lots of people don't. Which is why the strategy lets the opening range form and does nothing until it's finished, which usually takes about 20 minutes.
But don't take my word for it. Review the first half hour of the last thirty charts. One thing you'll find is that it is extremely rare for a gap to fill and rocket ahead without pause. Even today there was a lag of nearly 15 minutes after the gap was filled before price resumed the advance.
If you're going to play the gap at all, you have to decide whether or not you're going to fade it once it fills. If you're not, then all you have to do is place your 2pt entry stop above the opening high (or below the opening low) and go on from there.
--Db