How to do "countifs" (like in Excel) in Gauss please?

Here is a procedure that will accomplish what you want:

new;
x = { 1.0000000 2.0000000,
    1.0000000 2.0000000,
    1.0000000 4.0000000,
    3.0000000 6.0000000,
    3.0000000 6.0000000,
    4.0000000 9.0000000 };

x_counted = countIfs(x);
print "x_counted = " x_counted;
proc (1) = countIfs(x);
    local idx, tmp, out, j, tmp_u, tmp_inner;
    local i;
    
    out = {};
    i = 1;
    
    //Keep looping until we have removed all elements from 'x'
    do until scalmiss(x);
        //Grab chunk of first row matches
        tmp = selif(x, x[.,1] .== x[.,1]);
        
        //Remove chunk of first row matches from 'x' for next iteration
        x = delif(x, x[.,1] .== x[.,1]);
        
        //Grab unique elements from second column
        tmp_u = unique(tmp[.,2]);
        
        for (1, rows(tmp_u), 1);
            //Grab all rows where the second column is the same
            tmp_inner = selif(tmp, tmp[.,2] .== tmp[1,2]);
            
            //Create output row 
            out = out | (tmp_inner[1,.] ~ rows(tmp_inner));
            
            //Remove rows with this iteration's second column value from 'tmp'
            tmp = delif(tmp, tmp[.,2] .== tmp[1,2]);
        endfor;
    endo;
    retp(out);
endp;