Hi violent,
GAD65 is a test to see whether there is a specific antibody in your blood that is destroying your beta cells, I know this because I have them, hence my diagnosis of type 1.5, it will probably just be to see how quickly the disease is progressing if it is those antibodies that are causing your diabetes. Sorry to hear you're feeling so knackered, I was exactly the same and I agree with the others, discuss your symptoms to see if there is anything else they can do for you or there may be something else like anaemia or something which could be contributing.
Diagnosed with GD in 2010, Completely disappeared postpartum. Re-diagnosed December 2012 with type 1.5 diabetes, age 26, BMI 23 currently controlled by only Metformin, 500mg twice a day.