Hello Randall,
Thanks for writing in! Be advised that the Grid element is using the CSS Grid Layout which you can learn more from here: https://www.w3schools.com/css/css_grid.asp. The Masonry Layout on the other hand is a result of the popular Masonry Javascript library. Check it out here: https://masonry.desandro.com/
With the Grid element, you must make sure that the image element or any element inside the white cells do not have margins so that it will not exceed the cell area. Normally with the grid element, all of the cells are aligned to another. I am not just quite sure why your layout is different. Would you mind providing us the URL of the page where we can check your Grid element?
Best Regards.