You can definitely decline the glucose test, or ANY test or procedure you are not comfortable with however as a private entity they can also decline to see you. You can request a A1C test during your first trimester, but since youre past that you can ask for a fasting glucose test to get a baseline, and then perform an at home glucose test. This may be for as long as a week, depending on what your clinic requires but it doesnt include any gross sugar drinks that will send your system into overdrive. You test your blood sugar multiple times a day and record results
You can explain to your providers that due to your diet and lifestyle the traditional test is likely to show a false positive again, and also make you sick. You will need to advocate for yourself but there is no reason that you cannot do the at home test.