You need to do some processing in the case that the table data is all numerical, because the column header is returned in a cell array. The following example uses a uitable, but should apply to the table you have in your GUI as long as you have the handle.
h=uitable('Data',rand(2,3),'ColumnName',{'a','b','c'});
Data=get(h,'Data');
ColumnName=get(h,'ColumnName');
NewData=mat2cell(Data,ones(size(Data,1),1),ones(size(Data,2),1));
CombData=[ColumnName';NewData];
xlswrite(FileName,CombData);
