First and foremost . . . no matter how important this test seems right now (and, yes, I do know how important tests are), it is just one test of many . . . now, that is not to say that you do not prepare, but put the event in perspective . . .
Second . . . as you prepare for the test, focus on the important items . . . the best way to prepare is to do it as the course goes along so the test is not the * big * thing, but, and I was a student (actually still am) and I know sometimes that does not work out . . .as you study, identify the issues you need to cover . . . be selective . . .
Third . . . every day make sure you do some type of exercise . . . it does not have to be running a marathon . . . get out and burn off some anxiety . . .
Fourth . . . eat well . . . no, not junk food because it is quick . . .
Fifth . . . sleep . . . now, and this has worked for me and I don't know why . . . but I read over a chunk of info before sleep and for some reason I tend to remember it . . . .
Last . . . no matter the result of the test, chalk it up to experience . . . if you do great, remember what helped you to succeed; if you don't do as well as you would like, ask yourself what you need to do in specific terms . . . not just "I have to study more" but "after ever class, I need to review my notes" or "I need to ask for help or look for a study buddy", etc.
Test results do not define who you are or what you do . . . I had to take major (and these were BIG) exams and I cannot remember what I wrote . . . yes, sometimes things are attached to exam results like grades, but ask yourself . . . who would you rather be . . . a person with extreme GAD and good grades or a person with GAD being managed and average grades . . . . it always comes down to choice but I vote for less anxiety . . . .
If you can, try some mindfulness techniques to slow down your anxiety and, as always, if you have an official diagnosis of GAD, remember your school must provide due accommodation if you need more time or a quiet exam place, etc. but you do need to let them know in advance . . .
Will be thinking of you writing your exam . . . if you can, let us know how you are doing with the GAD --- the exam, well, it has value but your health is much more important . . .take care, kc