How do I create a drop-down that lists categories?

I want a drop down field to display a current list of categories on the site, much the way http://www.gravityhelp.com/documentation/page/Dynamically_Populating_Drop_Down_Fields explains.

I do NOT want to use the Post Field - Category field because I do not want to create posts, but rather people fill out a registration form and need to be able to select a category.

I haven't been able to nail down whether I would alter the code in the article above or use a value in Allow field to be populated dynamically > Parameter Name; and what exactly that code should be.

If anyone can point me to specific syntax for this, that would be great.