I heard somewhere that ground orange peel, applied to the soil around plants, will deter pests, namely some insects and cats. I have prepared a large quantity of ground grapefruit peel and would like to use it similarly. What do you think?

All citrus products, homemade or commercial, are natural pesticides for soil or foliar use.

However use all strong pesticides with caution and moderation.
All pesticides kill something, even some beneficial microbes in most cases!

As a sustainable gardener always remember to feed and attrract beneficial microbes and larger organisms first, and kill anything as a last resort.

Sidenote: GSE or Grapefruit Seed Extract is known for some health properties. Purifies drinking water. It is commonly used to treat stream or river water or when traveling in order to eliminate parasites or unhealthy microbes. (Military or hunters or travelers often use it.) Whole Foods has it. It can help knock out a case of food poisoning like Salmonella. Fast acting. If you have parasites in your gut (and this is quite common) it will help to eliminate them...and you will know it because of some cramping the first time you try a few drops. Later you will feel great! Occasionally, a few drops in a glass of water can be refreshing and revitalizing.