The water needs to be boiling to set the gel, but you probably don't want to heat up any fruit or frozen ingredients you plan to add. This is the time to add any mashed fresh fruit, blood worms or frozen brine shrimp.
Avoid feeding your fish too many fruits with a high sugar content. If you have a pond, you might enjoy watching them play with a slice of orange occasionally. The vitamin C is healthy.
A small amount of fruit incorporated into the gel food probably improves the taste. However goldfish don't digest sugar very well and too much fruit can cause digestive issues. If you think your fish has constipation or other digestive disorders you may want to avoid fruit entirely.
Think babyfood here, and remember to remove any skins and mash the fruit well so that there are no chunks larger than their mouths. Here is a list of fruit that is safe to feed your goldfish in small amounts:
This recipe makes 2.6 pounds of fish food (1180 grams) which fills three16 oz containers for freezing. Only 4 ounces of that is fruit, or about 10%. In this case the fruit was a baby food jar of apples and apricots and no fresh fruits were used.
In this recipe I used four small blood worm packs as shown above, thawed briefly and chopped with scissors. I used 15 grams of frozen brine shrimp or about one quarter of the package shown above.