"The Pristine Method" had a pretty good system of rating and trading gaps, imo. Basically, the founders tried to create a semi-objective way of rating gaps based on 5 criteria, and then using prescribed criteria/patterns for entry based on the gap's rating -- essentially what you might do if you were attempting to create an algo for trading gaps. The approach is based primarily on continuation of the trend ignited by the gap, although there are techniques for fading certain types of gaps, and for trading other special types of gaps.
The original Pristine service was bought out by T3 Live, and is now called the T3 Live Strategic Day Trader. They produce a lot of free educational videos, as did Pristine... as well as the over a dozen trading services that were formed by former Pristine educators & students. Try Googling for the many free videos, webinars, articles, etc. produced by these various services and see if they might appeal to you. I don't think many of the Pristine-based traders adhere strictly to the original system now, but they still use the basic concepts.
I've seen one trading service that taught setups based on gap fills -- e.g., "gap fill & go" -- although I'm sure there are more. Ironically, they also came out of Pristine. But those were also based on specific types of gap, and not a blanket rule-of-thumb that "gaps tend to fill" (because I don't believe they do -- especially not the strong gaps, and especially not within a daytrading timeframe).
