On a harp you can find 4 kind of strings:
All harps with large tonal range use wound metal strings for the lower bass range.
However there are small bardic harps, that are stringed completely with metal strings. Metal strings show a long sustain and therefore ask for special playing techniques with a lot of damping of the sounding strings.
Natural gut are the historical strings in the middle and higher tonal ranges. Today natural gut still is the standard in the classical domain on concert harps. They have a nice, warm sound, but are fragile and break more easily than nylon or carbon strings. And the price for quality gut strings is considerably higher.
Nylon is a synthetic filament, cheap and durable.
Only problem: Nylon strings tend to clash when played with energy.
Lever harps used to be stringed with nylon strings, and you´ll find lever harps with low tension strung with those strings .
Concert harps do not show large variations : length and calibration of the strings is quite uniform by models and manufacturers,
and the string tension on modern concert harps usually is quite high.
On lever harps there are very different concepts: you will find small lever harps , strung with nylon, with very low string tension, as well as lever harps strung with natural gut the same diameter and tension than the concert harps. .
We prefer middle tension , not too low. so you can play with energy and perform in a higher dynamical range. Therefore we use fluorcarbon strings on the harps we build , gut strings are available on request.