Gets or sets a value that specifies whether the column takes part in grouping and at which level.
An integer value that specifies the column's position among grouping columns. -1 if the GridView isn't grouped by the values of the current column.
The GridView supports data grouping by an unlimited number of data columns. When grid data is grouped by the values of a given column, the column's GroupIndex property is set to a non-negative integer. If several columns are involved in grouping, the GroupIndex property specifies the grouping level. If set to 0, the column is at the root grouping level.