I don't use Repashy, but I make a lot of my own gel foods using numerous natural ingredients such as whole fish or fish filets, insects, earthworms, duckweed, dandelions and numerous others; availability of these things is usually seasonal, so each batch is unique. Rather than mixing in vitamin powder I have on occasion added VitaChem, and my foods always contain large amounts of commercial fish food flakes, pellets, etc. which are marketed as being nutritionally complete themselves.
Gel foods are stored frozen in small individual portions. Ingredients are finely chopped and mixed in a standard kitchen (dedicated!) blender. Use just enough gelatin to hold the stuff together; you want food, not some messy quivering blob of gelatin with a few particles of edible stuff embedded in it. Trial and error is required to get the right proportions. Mixing the gelatin using meltwater from frozen fish foods lets you "reclaim" much nutritional value that would normally be wasted.
Natural foods, both animal and vegetable, must be collected from locations that are known to be free of pesticides or herbicides. A small residential backyard in the city...frankly, any location within an urban area...doesn't qualify.