Uncontrolled GD can lead to an unusually big (for that woman) baby, and "they" define big in this case (called "macrosomia") to be anything more than 8 lbs 13 oz. So technically, your baby was right under the "macrosomia" limit.
But seriously, 8 lbs 12 oz is not unusually huge or anything, and I would not suspect GD just because you had that size baby. And many women naturally grow big babies (including 12+ lbs). Big baby doesn't necessarily mean GD, but they do like to test for GD if you've had a big baby because a big baby *can* be caused by uncontrolled GD. I don't see any reason to test you twice though, based on an 8 lb 12 oz baby. Technically, by "their" definition, your baby was not "big".
And like PPs, I've declined the GTT altogether and tested my own blood sugar at home instead. I probably grow similar sized babies to yours (my DS #1 came early, but was big for his gestation... DS #2 is measuring ahead in size as well, both u/s weight and fundal height, and dates are correct). I also have a diabetic father and diabetic uncles. I started out pregnancy 50 lbs overweight. I have risk factors for GD. But based on my blood sugar testing at home, I very obviously don't have GD - my blood sugar has ranged from 91-95 2 hours post-meal, even with very diabetic-unfriendly foods. And it makes sense that I might have a 9 lb baby... my DH was 8.5 lbs 2 weeks early, and he's 6'5" now. I'm 5'9". Our babies aren't going to be short, and length adds weight!
Also, I have a build that easily supports a bigger baby, so that's what my body grows!