Good for you for pursuing this! The human body has an amazing capacity for keeping itself healthy -- in spite of our eating habits and our infrequent exercise and the vast sea of chemicals in our environment. But when things do get unbalanced, there's always a reason. Sometimes doctors forget this, and look only at the symptoms. Your blood test says that your B12 numbers are up, so therefore we've solved the problem.
If you are going to be tested for celiac, then it's important that you don't change your diet before the test. A few months of a gluten-free diet can allow your body to heal the intestines, and any anti-bodies will disappear from your blood, so that a blood test will also not show anything. After the tests at the gastro, if they show negative to celiac, you might still find it interesting to try a gluten free diet to see if it makes any difference. My tests came back negative because it took so long to get anyone to believe that it was appropriate to test for celiac in the first place ("if you had celiac, you'd have diahrrea several times a day!"). My acid indigestion was so bad that I was desperate to find out the offending food -- so I narrowed it down to gluten and worked on how to have a gluten-free diet. By the time I found someone who was willing to give me the tests, several months had gone by, and my body was feeling much better by that time. Negative tests for celiac, so I'm not officially diagnosed. But I'm 100% positive that I react in some way to gluten, so I'm on a gluten-free diet for life.
I don't mean to imply that you have celiac. But in order to test for it, you have to continue to eat gluten.
Please report back what you find at the gastro doc. We can all learn from each other.