It's difficult to replicate the exact difficulty and adaptiveness of the official practice exams (which are made up of real GMAT questions).
That being said, practice tests are a scarce resource (especially for the Focus edition). So having a precise strategy for using different sources for practice exams is important:
1. Use the official exams later in your prep (when you've got a solid conceptual foundation and you're focused on improving things like anxiety management, timing and test taking strategies like skipping questions).
2. Use other solid sources like GC tests for getting yourself into the mindset of the exam -- being under time pressure, dealing with rapid shifts in topics, difficulty and time pressures.
Almost everyone I work with tells me they've run out of official practice exams -- and it's because the incorrectly used the official ones. Use them for fine tuning your performance not for getting used the structure and pressures of the exam.