https://drafts.csswg.org/css-grid-3/#masonry-slack says that the initial is masonry-slack: 1em.

I think that the correct masonry-slack slack value is very dependent on the contents of the masonry, so 1em may not work well in some cases, and will probably just cause unnecessary confusion to authors.

I think it's better to default to 0, and then let authors choose what works best for them.