Adderall is a prescription drug, which means it's illegal to use it if you don't have an Rx for it. Aside from that, it does not do anything for you if you don't have ADHD. It's a stimulant, and if you don't have ADHD, it will just hype you up. The ADHD brain works differently, and stimulants help it focus.
Not to mention there are side effects that can be scary, such as muscle tremors. A family member of mine has ADHD and has tried a number of different meds to get the right med and the right dose, and developed a wicked muscle tremor after using Adderall. He switched to something else a month ago and still has a slight tremor. Do NOT mess with this stuff.
Don't cheat to get your desired score. STUDY for the GMAT and take it honestly. There are far better ways to improve your concentration, including eating more protein and fewer carbs, especially on test day.