Gap colours are controlled by the Stroke Panel. Window>Strokes
Hard to know what you mean, GAP colours for strokes that have double or tripple lines, or for dotted strokes. Not really used in tables, I believe.
But I think you are taking about Cell Strokes, and it's not the Gap Colour, that's a different thing.
Instead, in the Strokes Panel, when you have a row(s) or column(s) or cell(s) selected, choose which side of the cell(s) you want to adjust, anything highlighted in Blue will be what is changed (clicking them unhighlights them).
From here go to the Swatches Panel (Window>Swatches) and change the stroke colour to None.
(you should see two small squares at the top of the Panel on the left, the one on the top left is Fill Colour and the one to the right is Stroke Colour – select the stroke colour and change the colour to what you need, which in this case is None).
Hope that helps (it might sound confusing
sorry if it does)