"Performance with Gini Index"

Would I be correct in my assumption that I can abuse the Weight by Gini Index to use it as a way of calculating the performance of a model? 
If not, why not?  Here's a sample process demonstrating the abuse I mean. 

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<process version="6.4.000">
  <context>
    <input/>
    <output/>
    <macros/>
  </context>
  <operator activated="true" class="process" compatibility="6.0.002" expanded="true" name="Process">
    <process expanded="true">
      <operator activated="true" class="retrieve" compatibility="6.4.000" expanded="true" height="60" name="Retrieve Sonar" width="90" x="45" y="75">
        <parameter key="repository_entry" value="//Samples/data/Sonar"/>
      </operator>
      <operator activated="true" class="split_data" compatibility="6.4.000" expanded="true" height="94" name="Split Data" width="90" x="179" y="75">
        <enumeration key="partitions">
          <parameter key="ratio" value="0.5"/>
          <parameter key="ratio" value="0.5"/>
        </enumeration>
      </operator>
      <operator activated="true" class="k_nn" compatibility="6.4.000" expanded="true" height="76" name="k-NN" width="90" x="313" y="30"/>
      <operator activated="true" class="apply_model" compatibility="6.4.000" expanded="true" height="76" name="Apply Model" width="90" x="447" y="75">
        <list key="application_parameters"/>
      </operator>
      <operator activated="true" class="performance" compatibility="6.4.000" expanded="true" height="76" name="Performance" width="90" x="581" y="75">
        <parameter key="use_example_weights" value="false"/>
      </operator>
      <operator activated="true" class="select_attributes" compatibility="6.4.000" expanded="true" height="76" name="Select Attributes" width="90" x="648" y="210">
        <parameter key="invert_selection" value="true"/>
      </operator>
      <operator activated="true" class="set_role" compatibility="6.4.000" expanded="true" height="76" name="Set Role" width="90" x="782" y="165">
        <parameter key="attribute_name" value="prediction(class)"/>
        <list key="set_additional_roles">
          <parameter key="confidence(Mine)" value="regular"/>
          <parameter key="confidence(Rock)" value="regular"/>
        </list>
      </operator>
      <operator activated="true" class="weight_by_gini_index" compatibility="6.4.000" expanded="true" height="76" name="Weight by Gini Index" width="90" x="916" y="120"/>
      <connect from_op="Retrieve Sonar" from_port="output" to_op="Split Data" to_port="example set"/>
      <connect from_op="Split Data" from_port="partition 1" to_op="k-NN" to_port="training set"/>
      <connect from_op="Split Data" from_port="partition 2" to_op="Apply Model" to_port="unlabelled data"/>
      <connect from_op="k-NN" from_port="model" to_op="Apply Model" to_port="model"/>
      <connect from_op="Apply Model" from_port="labelled data" to_op="Performance" to_port="labelled data"/>
      <connect from_op="Performance" from_port="performance" to_port="result 1"/>
      <connect from_op="Performance" from_port="example set" to_op="Select Attributes" to_port="example set input"/>
      <connect from_op="Select Attributes" from_port="example set output" to_op="Set Role" to_port="example set input"/>
      <connect from_op="Set Role" from_port="example set output" to_op="Weight by Gini Index" to_port="example set"/>
      <connect from_op="Weight by Gini Index" from_port="weights" to_port="result 2"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="36"/>
      <portSpacing port="sink_result 2" spacing="36"/>
      <portSpacing port="sink_result 3" spacing="18"/>
    </process>
  </operator>
</process>