The GAD test is only testing for the antibodies. I don't think it tells you anything beyond the fact that you have the antibodies.
As for what is contributing to this. Not sure what you mean by "this". If you mean the GAD positivity, no one knows why you get the antibodies, it's possibly some combination of genetics and environment, honestly no one knows. If you mean the diabetes, well the GAD test tells you that those antibodies are killing off your insulin producing beta cells and that's why you are diabetic.
Don't look for too much meaning it - a GAD test just tells you you are type 1. I really dont think it is run for any other diagnostic purpose other then to figure out what type of diabetes you have.