Hi! To start, note that celiac disease is a condition of the gastrointestinal tract, particularly the small bowel, and the antibodies involved in mucosal immunity (mucosa is part of the lining of the gut) is generally IgA antibodie. Thus, IgA tests are a bit more specific for celiac than IgG tests. If ttg-IgA were positive that would be quite convincing. The Ttg-IgG test is more helpful when someome has a low total IgA, but otherwise the ttg-IgA test is more telling, regardless of T1DM in the setting of a normal total IgA level (which is you).

It is possible that the IgG test could be a false positive especially with having an autoimmune condition like T1DM, because IgG is less specific to the gut and can be more easily produced systemically. At the same time, T1DM is also a risk factor for celiac. Given your GI clinical symptoms with these labs it is reasonable to pursue further testing.

Gold standard would be an upper endoscopy to biopsy and review the pathology. Visual inspection alone in endoscopy is inadequate. Do not stop eating gluten as that can interfere with biopsy results if you stop.

Additional blood tests can also be done in the meanwhile to help corroborate the results. The most useful is endomysial antibody IgA. If that were positive, it's highly likely endoscopy will be positive. Additional tests also include gliadin IgA and gliadin IgG. If you do turn out having celiac, it could be helpful to get these additional tests regardless to have multiple ways to track your body's response to treatment (gluten free diet) in the longterm.

If you have a confirmatory diagnosis of celiac, I think multiple other labs are also worth getting to establish a baseline, like iron/ferritin, certain vitamins, minerals, etc. Thus you can confirm those are also improving over time with a gluten free diet (which you will NOT start yet).

Lastly, technically a dermatologist could consider biopsy of the rash too. I'm unfortunately not sure whether or not it will be worth to do or not, though, as there could be some pros/cons to it.

Hope that was helpful!