Cat food is formulated with all the vitamins and minerals a cat needs and with the assumption that the cat would only drink water in addition to eating the food. So if you want to make gravy yourself, you don't need to worry about adding any more nutrients.
However, you must worry about not adding any harmful substances to the gravy. First and foremost those are salt, onions and garlic. Your cat should have absolutely 0 of them.
I can think of several methods to solve this problem (while also sparing your money):
Cat food shake
If she loves the taste of the Friskes cans so much, try blending the solid contents of the can with water like a milkshake. Store the shake in the fridge to keep it fresh, but let it get back to room temperature before feeding it. You can also pop it into the microwave for a few seconds before feeding, but it shouldn't be warmer than lukewarm.
Cat water fountain
She might simply not like her current water bowl all that much and therefore prefer the gravy. Cats are known to prefer fresh running water. Cat water fountains were designed to cater to those instincts. You need to refresh the water each day. If you have chlorinated tap water, you should the water stand in an open, wide-mouthed container (like a bowl) over night to remove most of the chlorination. It's mainly the smell of chlorine that puts cats off.
Home made broth from food waste
This option is only viable if you regularly cook food from fresh ingredients or buy fresh or frozen bones.
Collect any raw meat cutoffs, skin, sinews, cartilages and bones (including fish bones and fish heads) in a sealable container and put it in the freezer. Every time you cook, you can add fresh cutoffs on top and return the container to the freezer.
Then, once in a while you dump the contents into a pot, generously cover it with water and let it simmer for 3 - 4 hours. The bone broth you get is very nutritious and hopefully to your cats taste. Store it in the fridge or freeze it in smaller portions. If it gets gelatinous, heat it slightly before feeding.
Please take note that you shouldn't add any seasoning at all. You also shouldn't use seasoned cutoffs like marinated meat or hot wings. Also, you must not feed cooked bones to your cat because they pose a health risk. Only feed the broth and maybe the wobbly bits (skin, cartilage and the like).