To see the gap you need Chrome 55 or Firefox.
A grid with a width of 90%. Six column tracks of 10% each, 5 gutter tracks of 2% each.
The grid-gap property controls columns and rows but as the grid has no height, the row gap resolves to 0.
If we give the grid a height, there is something for 2% to be a percentage of. So we get a gap.