//get the reference to the SelectionLayer
SelectionLayer selectionLayer = this.gridLayer.getSelectionLayer();
//select cell with column position 2 and row position 0
selectionLayer.setSelectedCell(2, 0);
//freeze the first two columns
natTable.doCommand(new FreezeSelectionCommand());
//remove selection
selectionLayer.clear();

natTable.doCommand(new FreezeColumnCommand(natTable, 3));