So the official guide and most of the official resources are lacking hard questions simply because it’s always a pyramid you need a lot of medium and easy questions because that’s what 80% of people score and then you need just a small number of hard questions and a very small number of very hard questions.
That is the reason the official guide questions are generally on the easier side and that represents the composition of an average question bank for the exam but if you do well, you’ll be getting all the harder ones even though there’s a small number of them.
I always recommend all of the official resources though if I’m not mistaken I don’t think you’ll find that many hard questions in the question collections there’s still worth it for the verbal questions.
Some of the hardest questions I’ve seen or found in the gym prep the version before the focus edition. There are no official explanations as always but quite a few experts to put in their thoughts on GMAT Club.
On the real test, if you’re doing well you will be getting hard questions and you’ll be getting questions that you would’ve never seen in your life. You’ll have to apply math principles and skill that you’ve learned in the past but solving questions will not help you solve those questions at least directly. So if you’re doing really well on the gym mat, prepare to be stumped and have no clue how to answer certain questions.
PS. Make sure you don’t just do a lot of questions but you actually study the principles behind them and make sure you never make the same mistake twice. If you do then you’re wasting your time.
PPS. A lot of people lose points on the GMAT because they mess up the timing so make sure your timing strategy is being worked on.