Like in science, sport or any other field, the GMAT requires a mastery of a specific set of theory in order to do well. Doing well means being flexible enough to apply your knowledge in different scenarios and situations.
Consider sailing for example. Imagine that you take your first class one day. Typically, on a first class you learn about the different parts of a boat: Aft, starboard, sail, bow, rudder etc. You also learn about different maneuvers such as jibing and tacking. If it’s a pleasant day, perhaps you will go out on your first practical and with the help of your instructor, perform these maneuvers.
Now imagine that on the second day you feel confident enough in your ability and go out on your own. The weather however is not so pleasant. The wind and the currents are strong, and your instructor is not there to guide you. How easy will it be to navigate? Not so easy. One day is not enough to internalize the theory you learned.
To sail well, particularly in rough weather conditions, your knowledge of navigation basics must be very clear in your mind. You must know how to navigate, maneuver, and understand well the function of every part of the boat. Now is not the time to learn, now is the time to sail. The theory you learnt and the boat are tools that you need to arrive at your final destination.
This is similar to the GMAT in that your understanding of the theory must be nailed down long before your test date. A difficult problem requiring knowledge of square root multiplication rules for example is not about solving the square root, but about using the square root multiplication rules, your tools, to determine the answer. If you’re knowledge of theory is weak, you cannot solve difficult problems that have subtle differences from what you practiced. You have on average 2 minutes per question. These 2 minutes are for solving the problem, not for remembering what rule you need to use and how to use it.