In #2923 (comment) I've a proposal for multicol overflow in the block direction. This would create rows, similar to flex lines or grid rows, and the expectation is that the row-gap property would then apply here to allow authors to create spacing between rows.

column-gap: normal in multicol has a used value of 1em (it's 0px for flex and grid containers), the question here is whether row-gap: normal should also be 1em for multicol or whether it too should be 0px?