Often the best choice for beginners and those who like the nylon tone and feel. They are easier to keep in tune, cost less, and are more forgiving of technique than gut. They have a beautiful tone and are used on my custom harps with from low to high tension string sets.
I use bass wires, normally made from steel/fiber/ bronze for the lowest octave on my larger model harps, and then a series of wound nylon/nylon and sometimes steel/fiber/ nylon as transition strings to the bass wires. The transition from bass wires to the nylon is very gradual by using the nylon/nylon and steel/fiber/nylon transition strings, and easy on the ears.
For my Custom Harps designed to be strung with Gut strings I offer two general types of gut gauges and string tensions. Your custom gut strung harp can be designed to have a custom/personal variation of tension that suits your needs and playing style perfectly.