Welcome to the forums. Sorry to hear you're struggling with this problem.
I have to admit I was unfamiliar with Granuloma annulare so had to do some research but I think I can now offer a few suggestions with a fair degree of confidence.
First of all how was your blood sugar tested? A single blood test is practically worthless. The only way to see how your body handles sugar is to undergo a glucose tolerance test. This involves drinking a glucose solution then having your blood sugar measured every 30 minutes to an hour over a period of usually 4-6 hours. Many fibromyalgia specialists including Dr. St. Amand and Dr. Teitelbaum consider blood sugar problems to be a major issue in fibromyalgia. If there is a problem it can be treated with nutritional therapies, see this page: Hypoglycemia & Insulin Resistance
Second it seems that Granuloma annulare is associated with numerous auto-immune conditions including fibromyalgia and lupus so anything that improves immune function is likely to have an effect on the condition. I will try to look into this more specifically for you but it's an area you can look at yourself too.
Something that I am confident will be beneficial is a good quality fish oil supplement containing high doses of an omega-3 fatty acid called EPA. This helps reduce overactive immune responses and control inflammation and is also beneficial to skin health, improving moisture content etc. The omega-6 fatty acid GLA found in evening primrose oil and borage oil is also beneficial for skin health. Look for these at your local healthfood store or check out our store here at The Environmental Illness Resource: Omega-3 & Omega-6 Fatty Acid Supplements
Hope this helps and you find some relief soon.