ASX Morning call 17th July, page-35

Howdy: Actually, a gap is a gap, whether it's an inside gap, or an outside gap. Inside and outside gaps are terms used for stock gaps, but the general rule does apply for index gaps too, that inside gaps will fill quickly, and outside gaps may not fill quickly. Inside gaps tend to be smaller and outside gaps tend to be larger, so for SPI Futures, the inside gaps would be the common one pointers that occur due to the small bid/ask volume, and the outside gaps would be the larger at the open gaps, but in Futures world, the outside gaps fill more quickly than outside gaps on stocks.

Some general information is here:

https://www.tradingacademy.com/lessons/article/gap-play-2/