The command increases the size of a
selection in the current image. It works in a similar way to the
Shrink command, which
reduces the size of a selection.
A Peculiarity of Rectangular Selections
When you grow a rectangular selection, the resulting selection has
rounded corners. The reason for this is shown in the image below:
Why growing a rectangular selection results in rounded corners
If you do not want rounded corners, you can use the Rounded
Rectangle command with a 0% radius.