Hi Andres,
Every Number Property rule that you will ever see on the GMAT can be dealt with by TESTing VALUES. That approach will often lead you directly to the correct answer OR help you to define the pattern(s) involved in the question so that you can answer the question in some other way. So while you can certainly memorize all of the common Number Property rules that could show up on the GMAT, you can also deal with them in a relatively fast and practical way on a case-by-case basis.
Here's a simple example: What happens when you multiply any two ODD numbers together? You don't have to memorize this information - just prove it, right now... (hint: TEST VALUES)?
GMAT assassins aren't born, they're made,
Rich