In GdNi5, the muon localizes at the 3f interstitial site and below similar
to 80 K a second muon site becomes populated, the so-called 6(ring) site. T
his ring site is metastable and the muon hops to the 3f site. We determine
the mean time of stay of the muon in the ring site and show that it is gove
rned by a multi-phonon quantum diffusion process. The coincidence energy is
measured to be E-a = 272(10) K and the tunneling matrix element J = 0.11 (
2) meV.