I am 31 weeks with baby #2. I tested myself early and had high readings around 5 weeks but even though they did a fasting blood test for me around 7 weeks and GTT around 16 weeks I passed them both. Failed second GTT at 26 weeks. Some trusts diagnose you from your own high readings, some treat you like you have GD straight away because you did the first time, it really varies. My problem was without diagnosis I didn't have the supplies to test myself that long so I just ate like normal until my failed GTT. It's up to you though.
It’s not guaranteed but I have a friend who was in the same boat, and she did a pregnancy food program to keep her numbers steady during her second pregnancy and ended up not getting it on the second when she had it on the first. So it’s possible!
There is no guarantee, but it is more likely than not that you will have it. Eating on the diet can't hurt, so if you're comfortable to start you could. Tracking your sugars won't tell you if you have it or not unfortunately only if you are spiking at that time. Also the GD numbers are intentionally low so your blood sugar is more consistent and baby is kept safe, but numbers higher than that on a non-consistent basis aren't necessarily harmful. Your doctor will probably recommend an early GD test and if you're negative do another at the normal time.