How to compute Gini impurity in random forest (treebagger)?

Hello. I am doing classification using treebagger random forest. I should compute the gini index or gini impurity to understand each feature importance in classification. Can any one help me with that? thanks. Here is my random forest classifier code:

nTrees=50;
B2 = TreeBagger(nTrees,train_data,train_label, 'Method', 'classification');
predChar2 = B2.predict(test_data);
predictedClass2 = str2double(predChar2);