  The Mikheyev-Smirnov-Wolfenstein (MSW) explanation of the solar neutrino
problem is currently compatible with three distinct regions of the two-neutrino
oscillation parameter space (delta m^2,sin^2 2theta). We focus on the region
with the lowest value of delta m^2 (~10^{-7} eV^2), which implies significant
Earth regeneration effects for low-energy solar neutrinos. We point out that
such effects are not only observable as day-night variations of neutrino event
rates in the real-time BOREXINO experiment, but also as seasonal variations in
the radiochemical Gallium Neutrino Observatory (GNO) at Gran Sasso. We present
detailed calculations of the difference between winter and summer rates in GNO
(six months averages) in excess of the trivial seasonal variation due to the
Earth orbital eccentricity. We show that, within the low-delta m^2 MSW
solution, the net winter-summer GNO rate difference amounts to 4-6 SNU, with a
dominant contribution from pp neutrinos. We also give analytical expressions
for the winter and summer solar exposure functions at the Gran Sasso site.
