I have been practicing using the Quant portion of the GC tests, and I have seen a significant improvement in my understanding. The GC tests have a been a tremendous resource, specially as a source for upper quant level questions. These questions really push the limits in your understanding. Besides official material, these questions are certainly the next best thing available. I would recommend to anyone wanting to break through the lower 40's level of the quant score. I started in the low 30s; however, through consistency and persistence, I have crossed reached a 45 quant score. Best of luck in your journey!