Here’s another way to do it, to add to the collection:
#grid(
columns: 3,
gutter: 5pt,
..2 * (grid.cell(x: 1)[$->$], ),
[A], [B],
[C], [D],
)
How it works: We can insert grid cells at fixed coordinates. We insert 2 grid cells at x: 1 (second column). We need to specify the number of rows (2).