Gmac mock tests means the Official Practice test. Real is the real one you give at the test center or online to get the official score.
On average the official mock/practice tests are considered pretty close to the real one. You do need to understand the adaptive nature of the test - so it gets more and more difficult as you get more and more questions right. So in a way if you find the real test more difficult, that means you are doing better on that test.
The similarity of the score algorithm and question difficulty tag (since each retired question and each live official question has been tested on thousands of test takers, and there is immense statistical inference power in large numbers) ensures,” that, on average, you will get the same difficulty level of question at the same score performance level.