I would say that the GMAT quantitative has two different types of difficulty. One type is what you are probably thinking of...difficult geometry problems, complex word problems, probabilities, etc. Many of these can be at the higher levels, but other questions that are frequently missed by a majority of test-takers involve simple concepts, such as positive/ negative and odd/even. These questions - frequently data suff. but not always - are designed to capitalize on your assumptions. People miss these questions not because of their lack of knowledge but because they do not have a proper strategy.
My advice to you, get a good strategy to avoid making mistakes (answering the wrong question, misreading, etc.). This way you will know as you practice that the questions that you miss are from a lack of math knowledge.
You can get high-level questions right if you combine three things: knowledge from study, a methodical strategy to avoid missing questions and making assumptions, and finally a little flexibility and imagination to tackle unfamiliar problems.
And if you are looking for some difficult quantitative problems - especially in terms of the actual concepts, the Veritas books now available at Amazon have many questions that are at the higher difficulty levels and can supplement the very few questions in the Official Guides that are really difficult from a math knowledge perspective.
Good Luck!
