Basically I have a form with a  that chooses which set of data to use (values are \"m\", \"f\" and \"c\"). I then have a dictionary/object with th         
        
You don't have "mode" attribute in that variable. You must use if's to detect which sex you are processing and get gdas.m.fibre or gdas.f.salt.
Since you're referencing the property via a variable, you need the bracket notation.
var gda_set = gdas[mode];
...which is the same notation you would use if you were passing a String.
var gda_set = gdas["f"];
You can use gdas[mode], it selects the element that is indexed by the value of mode.