Why there are no tutorials on calculating Gini impurity in a three-valued categorical feature variables? Trying to learn here so please go easy on me if I asked dumb questions.

As the title says, I was searching for a tutorial that calculates the Gini in a CART algorithm for a three-valued feature variable. (i.e., assuming we have a feature variable named, "Blood pressure" and it has three levels (low, medium, and High)). However, I didn't find one. A perfect example of that is the Play Tennis problem, we can have a solution using the entropy method, but not with the Gini method.
There are no categories, you encode these columns as integer ones and proceede as usual.
Gini is there just to help you make decision on how and where you split. Gini,Entropy,variance are all replaceable components.