I guess I've gained a bit of a reputation as the goldfish gal around here.
Like little children, a lot of goldfish aren't fond of greens. I've noticed this especially with the younger ones.
I guess it's the same for children of all species! If you're planning to do gel food, I wouldn't worry about getting veggies in now. He'll get them soon enough.
I'd be more than happy to share my gel food recipes. I've made a few batches now and have evolved my recipe from what I was initially given. Now that I feed gel, I've completely eliminated pellets from their diet. There just isn't a need especially when I have fish that already have and are prone to bloat and swim bladder issues. If you do want to give them pellets, NLS has a goldfish formula that's good. The only reason I would do something like that is if I was going away for more than 7 days (especially in the summer) and wanted to have someone else feed them. I hand feed, and it's hard to get them to trust another person.
Now for my recipe: (I like to use babyfood aka already blended food because it makes things easier)
3 jars baby food, green veggies (only 1 sugary)
1/2 cup boiled hot water
2 envelopes of unflavored gelatin
1/2 adult multivitamin (no iron)
1/2 acidophilus tab **
1/2 calcium carbonate tab **
1 1/2 oz salmon (drained if from can) roughly half a can
1/4 cup finely chopped veggies like spinach or kale ***
1/2 teaspoon garlic powder ***
Dissolve 2 envelopes of gelatin in 1/2 cup hot, boiled water. Mix veggies, salmon and crushed pill stuff in a small bowl. Pour gelatin/water mix into the bowl. Stir well. Pour into a flat container so it's about 1/4" thick. Put in fridge to gel. Once gelled, cut into bite-sized pieces. (This is harder with young goldies, so I just cut them into large chunks that I cut pieces from nightly). Place a week's worth in a plastic baggy and freeze. Be sure gel lays flat.
** optional items; I don't add them as they are hard to find in my area
*** optional items that I do add
As young goldfish need more protein than adults (3+ years), I just bump up the salmon to 2 oz for your little guys until you notice a slowing of growth (pictures are a good gauge of this) or around 3 years of age.