Hi abybencent.
Here are some things that come to mind with regards to reducing and handling fatigued you may experience when taking the GMAT.
One approach you can take to reduce fatigue is to keep practicing and developing you GMAT quant and verbal skills. Simply put, the better you are at answering GMAT questions, the less fatigued you'll be at the end of the test.
For developing stronger skills, what generally works best is learning concepts and strategies first, then practicing untimed until you achieve high accuracy, and lastly working on speeding up to test pace.
A second approach is to take many practice tests to develop stamina and become accustomed to the test experience. The more practice tests you take, the more comfortable with the test experience you'll be and the better you'll feel toward the end of the test.
The final thing you can do is learn to get questions correct even when you're becoming fatigued. It's interesting how you can learn to remain lucid and get questions correct when you're fatigued. I noticed that I could learn to find sort of a different dimension of myself when I was fatigued and somehow maintain clarity. I went so far as to do practice questions late at night to learn to get them correct when I was tired. Learning to operate well even when you're fatigued can help you get the final few questions on your GMAT correct.