Thanks for mentioning that this is the "Gauss' Formula", I knew there was a formula to solve the problem, but couldn't remember it, and I'm not as smart as Gauss :D

It best fits under the 7 kyu description, which is:

Iterating arrays and returning a subset of values
Basic data type manipulations
Common operations such as sorting (using built-in sorting methods)

8 kyu kata are usually much simpler, dealing with stuff like simple function definitions, basic variable assigments, basic math operations, and basic syntax fixes

Why is this a 7 kyu kata? It's a really basic one....

I like the use of greetee as the greet function argument

Sweet

I agree with @awolfson. Check out Confident Ruby (http://devblog.avdi.org/2013/08/26/confident-ruby-is-finished/) for a more detailed look on how to structure code in a way to avoid checking arguments on multiple levels