If you have 3 columns in one line and change your section gap you might think that nothing happens. But this is because you don’t have any content in them.
The gap is used as left right padding for the child columns of your section.
The problem usually happens when a user wants to add a background color and still have a gap. This would work if the gap was used as margin, not padding. But this is not how it works.
So if you want a column with a background color and also gaps you will have to ad a column inside a column.