A form on a site using Gravity and how it was done

That is done with conditional logic. When you have a radio button or check box field, you can add a field after that that says "if the answer to {the question above} is equal to {whatever} then show {this}". It's very cool.

It's on the advanced tab and is called "enable conditional logic". I don't think it will appear unless there is a form field with radio buttons (multiple choice) or a check box. Those are the two field types that will allow you to create the conditions for your conditional logic.

Screenshot: http://i.min.us/iecZMq.png

I thought it was in the documentation but I am having a hard time finding it. New documentation is coming with the impending 1.5 release.