Not all gap-ups are created equally.
If the gap-up is on insignificant volume, this really doesn’t tell us much.
If it occurs on BIG volume, this is an indication of new institutional interest.
BIG volume should be at a minimum the largest daily volume in over a year.
This volume should stick out clear as day.
If you are second-guessing if the volume is big enough, it probably isn’t!
The range and close on the gap-up candle are also significant. If it can close in the upper 50% of its daily range the stock should be watched closely for consolidation.

If a stock fails to close in the upper 50% of its daily range on the big volume gap-up but holds its gap-up low, continues to consolidate tightly over the next week or so, and offers an entry point, this shouldn’t be ignored.
The daily 50% rule will help you weed out the majority of good versus bad big volume gap-ups, however, to be successful in the market you cannot be too rigid in your approach.

It is important that you take into account the health and trend of the general market. If it is a healthy bull market, big volume gap-ups that meet all of the criteria will succeed most of the time.
When you see a big volume gap-up, you ultimately want to see many more across the market in a general uptrend.
If this one big volume gap-up is an anomaly and the rest of the market looks terrible, do not try to outsmart the market.
Remember 3 out of 4 stocks follow the trend of the general market, so market direction will play a major role in your success here.

$SNAP & $PYPL gap-up examples