Northern Renaissance Instruments, UK suppliers of strings for period instruments, has an article on hurdy-gurdy strings on its website - as well as a wide variety of string sets for sale.
“Since the melody doesn’t cross strings In hurdy gurdies, choosing strings is a matter of finding the desired blend of sound, rather than balancing strings for equal output. Blend involves comparative loudness. The loudness of bowed strings is largely dependent on string tension. Different string tensions at the same pitch are achieved by different weights of the vibrating lengths of the strings.”
Worth a read here.