I am skeptical about the stuffs which are available for free; what is the credibility of GMATPrep™ software; how accurately it assesses my preparation level?

If any CAT simulation software one can trust, it’s GMATPrep™. How GMAT score is calculated is undisclosed by the official sources. But they have provided the tool to gauge the performance. GMATPrep follows the same algorithm as real the GMAT does.