The diameter dependence of the thermal conductivity of nanowires is usually modeled using Matthiessen's rule, by putting the mean free path of phonons equal to the diameter d of the nanowire. This results in a thermal conductivity κ that decreases with decreasing d, due to the increase in boundary scattering. Recent molecular dynamics studies of heat transport in thin silicon nanowires have shown a nonmonotonic diameter dependence of κ, where a decrease with decreasing d is followed by an increase to a value of κ exceeding the bulk thermal conductivity. This increase of κ was explained by an increase of the importance of hydrodynamic transport effects in the thinner wires, where the normal scattering by phonon-phonon interaction increases, but the Umklapp scattering decreases [Y. Zhou, X. Zhang, and M. Hu, Nano Lett. 17, 1269 (2017)10.1021/acs.nanolett.6b05113]. Here, we study heat transport in thin nanowires of the compound semiconductor gallium-phosphide in the wurtzite crystal structure, using molecular dynamics simulations. A similar nonmonotonic d dependence of κ is found as in silicon nanowires, but with a minimum in κ occurring at a much larger diameter of d≈8nm instead of 2-3 nm.