T1D is an autoimmune disease which destroys the pancreas' ability to produce insulin. Your body needs insulin to process sugar in the blood into energy, a large part of that sugar comes from carbs.
Celiac is an autoimmune disease which is a reaction in the gut to gluten. Exposure to gluten causes inflammation in the gut and ultimately damages the lining of the intestine to the point where your body cannot absorb nutrients (including the carbs) from the food you eat.
What bluemax observed is the damage from celiac was restricting his carb absorption which reduces the amount of insulin required to keep blood sugar in range.
It takes ~6 months abstaining from gluten for the intestine to heal and for carb absorption to return to normal, over which time insulin requirements would also return to normal. Going gluten free allows the intestine to heal (and you have to stay gluten free -- the celiac doesn't go away).
My daughter is T1D+Celiac so I'm quite familiar with it as well.
