You can call 'getValues' method to get all the selected values on the combogrid. To get all the unselected rows, please try this:
var cc = $('#cc');  // the combogrid object
var opts = cc.combogrid('options');
var values = cc.combogrid('getValues');
var rows = [];
$.map(cc.combogrid('grid').datagrid('getRows'), function(row){
	if ($.inArray(row[opts.idField], values) == -1){
		rows.push(row);
	}
});
console.log(rows)