Group Column Sum

Below is a procedure that, I think, will do what you want. It would be simple to extend it to also allow computation of statistics other than just the sum. Also, with GAUSS 17's new ability to load data from CSV and Excel files by variable name, you could also make it operate on a file and specify the 'id' and 'sum' columns by name.

//Starting panel
data = { 1 1 1,
         1 2 3,
         2 1 1 };

//Which column should be used to define the selected group
id_col = 1;

//Which column should be summed?
col_sum = 3;

//Create new panel matrix with additional column
//containing, sum of column 3 by group
data_new = grpSum(data, id_col, col_sum);

proc (1) = grpSum(data, id_col, col_sum);
    local out, groups, group_idx, group_var, group_sum;
    
    //Add an extra column to hold the sums
    out = data ~ zeros(rows(data), 1);
    
    //List of all groups
    groups = unique(data[.,id_col]);
    
    //Loop over each group
    for i(1, rows(groups), 1);
        //Find indices of the i'th group
        group_idx = indexcat(data[.,id_col] .== groups[i], 1);
        
        //Extract data to sum for i'th group
        group_var = data[group_idx,col_sum];
        
        //Reshape to correct size for write
        group_sum = reshape(sumc(group_var), rows(group_idx), 1);
        
        //Write sums in appropriate location
        out[group_idx,cols(out)] = group_sum;
    endfor;
    retp(out);
endp;