I only noticed a decrease in fatigue if I spread my carbs out. I always ate regularly, and not unhealthily, but before the diagnosis it'd be really carb heavy at times. Once I started spreading the carbs out, I felt better. 2-3ish serves at meals (~40-50g carb) and 1-2ish serves (~20-30g) at snacks, and I felt a million times better.

Not sure about the slow weight gain, I guess it depends on your pre-pregnancy BMI and healthy weight gain target? My pre-pregnancy weight was 57kg, and they wanted me at around 70kg, but at 28 weeks I was 65.5, and then I ended up losing it, and I'm only just now, 8 weeks later, almost hitting 67kg. They were concerned only because at this stage of the pregnancy is when the baby should be packing on the weight and I didn't have the weight to lose to sustain my energy levels and the baby's needs as well. But I do know if you have a bit more wiggle room (i.e. if you had a BMI >25 pre-pregnancy) they don't tend to stress about weight loss or slow gains in GD if you are eating right and your BSLs are controlled.