My understanding was the GAD test was an indication of type 1 (or not) but not definitive.
A quick internet trawl of "antibodies and diabetes" uses language like "suggests" and "are characteristic of" and "thought to circulate" and "commonly found".
Was there a reason why, after 20 years, your endo suggested you had the test to identify anti-bodies?
Have you been struggling to manage your diabetes? In the UK, the GAD test is not something done on most diabetes reviews.
What impact has your endo's diagnosis had on your treatment?
Sorry for all the questions in response to yours - I guess I am as confused as you by this.
I hope you get some answers (to your question - mine are less important).