As with most animals, nutrition is very important, but Gouldians are a little different to most grass finches.
Here in Australia, most Gouldians rely very much on seed. In comparison with other grass finches, they are not interested in most live food, even when feeding young, and also take relatively little in "greens".
So my mainstays are a finch mix (mostly millet, panicum and canary seed, little bit of oily seeds like nigar). Avoid oats and sunflower seeds, they do not eat them. Half ripe seeds, soaked seeds and sprouted seeds.
The half ripe or "green" seed can be either cultivated or harvested from "weeds"
My main source is a 3 m wide clump of green panic that grows in my garden. Try getting that into a pot !
It starts seeding about now, early summer, and seeds through to March, late summer. This is a problem because I breed my Gouldians from February to May, and much of the time the panic is not seeding.
Fortunately crab grass seeds in late summer, and usually a bit later than the panic.
Then there is a bit of a gap, before the winter grass starts in early winter and followed by the Algerian oats in spring
So I can usually harvest half ripe seeds from my garden for 8-9 months of the year, depending on how "lazy" I get with the weeding.
I have cultivated millets. commercial panicums, canary but the yields are not worth the effort ,when my weeds grow with no effort
Next best is sprouted seed, as mentioned in other posts recently a good sprout rate is critical, but this is easy to test for.
Unlike Mike Fidler I just use white millet only in my sprout mix, this avoids the problem of different seeds sprouting at different rates.
I add a vitamen powder to it (this is the only supplement I give my birds) and freeze it. This is the great beauty of sprouted seed, it can be frozen and be just as palatable on thawing. It also maintains all its nutrition, which is significantly greater than either dry seed or soaked.
My young Gouldians have just finished the moult, they are vibrant, sleek, energetic. The cock birds are glowing, there is not a feather out of place and they know it
Cheers
Brett