as i read from bunuel's msgs, some of these questions are from gmat prep, but not all. Nonetheless i have been solving these tests myself lately and the quality is good! you learn something new with every question.
as an update, i started doing the GMAT prep problems (someone dumped every single DS/PS for GMAT prep 1/2) and since I don't have much time my goal is to finish the entire gmat prep 1. ~400 problems.
For instance, the last 10-12 problems on GMAT PREP 1 Data sufficiency are very good and very tough. I went through some of the problems in Bunuel's sig before, etc. and they look nothing like the problems in GMAT prep. So I would highly suggest doing these problems since they are official problems.
I also did the first 1-40 (because I had no clue how the difficulty was ordered) and realized that I got maybe 5-6 extremely easy problems (i think maybe 400-500 level problems), and maybe 4-5 good hard problems (600-750). So it appears that the problems increase in difficulty but with randomness interspersed.