There are a lot of recipes available for making goldfish food. If you are so inclined, I've tried to collect a few here that I view as "proven"; that is, have been used by somebody for a few generations of fish.
Here is a well known gel food recipe and a variation of it that I have used. These have been used for red and white ranchu and are high in protein to encourage wen growth, and contain ingredients to enhance the red color.
If you need a real quick, simple way to produce a gelatin food for short-term use (such as for feeding an antibiotic or for floating problems), an easy approach is to buy a jar of baby food, add a packet of gelatin powder to it, and microwave until bubbling hot. Then let cool, cut into strips, and freeze whatever you won't use in a day or two. I would suggest using "strained peas and carrots" or "strained spinach" baby food.
Above: cooking up a big pot of gel food on the stove, to dissolve the gelatin.
Above: after setting overnight in the fridge, I cut the gel food up and it goes into the freezer, then into ziplock bags. Wax paper separates the layers.
Above: Sigh. Ninety minutes of work to make the food, and there are still half an hours worth of dishes to do.
For your interest I also have some recipes and information from the Far East on goldfish foods.