Pinning: Is the gap between the pinned and unpinned columns expected?.... YES

When you are pinning columns on the right side, if you other columns do not fill the space there will be a gap. You can circumvent this by resizing all columns, via gridOptions.sizeColumnsToFit()

This is the natural behaviour of pinned columns. If the left side columns don't have enough width, the right pinned columns would display after a gap (because they are pinned on the right) and are not responsible for the other columns' appearance.