  Scintillating crystal detector offers potential advantages in low-energy
low-background experiments for particle physics and astrophysics. The GSO
crystal is an interesting detector to explore for future neutrino physics
experiments. The contributions to background due to the various channels of
intrinsic radio-isotopes from the Th-232 and U-238 series are identified and
studied with time-correlation analysis and detailed fits to the spectral shape.
Good agreement is achieved between measured and simulated spectra, indicating
background suppression factors to the 10^{-2} - 10^{-3} level are possible. The
procedures can be adopted for background understanding and suppression in other
low-count-rate experiments where the dominant source of background is from
internal radioactivity. Based on 1656 hours of data taking, limits on the
double beta decay half-life for the various channels in Gd-160 are presented.
The limit for the neutrinoless mode is T^{0 \nu \beta \beta}_{{1/2}} > 2.0(0.8)
X 10^{20} y at 68(90)% confidence level.
