this.grid.InternalColumns.StyleInfo.CellValueType = typeof(Int64);If this is a GridControl, then make sure the grid.ColStyles[col].CellValueType is set to typeof(Int64).
this.grid.ColStyles[col].CellValueType = typeof(Int64);
If this did not work, PopulateValues must have set the individual cells styles to string. So, if that is the case, try:
GridStyleInfo style = new GridStyleInfo();
style.CellValueType = typeof(Int64);
this.grid.ChangeCells(GridRangeInfo.Cells(1, col, this.grid.RowCount, col), style, Syncfusion.Styles.StyleModifyType.Override);
to change the individual cell styles back.