I purchased an unweaned Grey before I knew it was such an issue

[newbaby]

Go back to what works for her. Take a photo of the formula after you mix it and post it. It may be fine the way you were doing it.
You shouldn't have to feed at night but if you think she needs it go ahead.
Not all Congo Greys are 500+ grams. She may end up being a smaller one at under 400 grams. What did the vet say about her growth and development?

She goes to the vet on Tuesday and I can ask then. I fed her at night because the breeder said to feed her every six hours. She’s wasn’t gaining weight though so I asked here if there was anything I could do. I’ll go back to the way I was feeding her until I can speak to the vet.

 

[rheashard2]

You've hit the nail. Feeding companion birds from hatching onwards is like a piecing a puzzle. Owners have to fit the pieces together. Many parrots are resistant to sudden changes in their diet. Can ease your baby into thicker formula? Thicken it over several days. As stated your CAG may be on the small side. It may be her norm to gain weight at a slower rate. Books give general advice. There's plenty of normal variation out there. Get an avian vet evaluation. It'll rule out a lot. My female CAG is on the low end of the average size. Combined with her high physical activity has me struggling to keep her weight in range.