RowSpacings is an option for GridBox which specifies the spaces in x-heights that should be inserted between successive rows.
The default setting is RowSpacings->1.0.
RowSpacings effectively specifies the minimum distance between entries in successive rows; individual entries will often not fill their rows and will therefore be further apart.
RowSpacings->n uses a column spacing equal to n times the height of an "x" character in the current font.
RowSpacings->, , ... can be used to specify different spacings between different rows. If there are more rows than entries in this list, then the last element of the list is used repeatedly for the remaining rows.