For the purposes of minimum night gap rules, those are targeting gaps where the available nights are less than the minimum nights for the period. The point of the minimum night gap rules is to make gaps bookable that otherwise wouldn't be bookable because of the min night rules by reducing the number of nights.
So if you have a 3 night gap and your min nights are 2 during that period, or 3, then a gap night rule will do nothing because the gap is not shorter than the minimum nights.
But if you have a 3 night gap and a 4 night min, then a 2 nights in gaps rule would reduce the minimum nights during that period. If you set the rule to use the length of the gap, at least 2, then you'd end up with a 3 night min because that is the length of the gap. Or if you wanted to override min nights to 2 regardless of the length of the gap, then it would end up with 2 in that case.
But either way, if you had, say, a 5 or 10 night gap in that 4 night min period, then the min nights in gaps would never apply because the min rule is not more than the gap length.
We don't currently have something like @Lydia mentioned where you can say "for a gap of this length, use that min" -- instead it's automatic based on what min you set and the gap being shorter than the min nights rule would otherwise be in that period.
