  We present the final data release from the GALEX Arecibo SDSS Survey (GASS),
a large Arecibo program that measured the HI properties for an unbiased sample
of ~800 galaxies with stellar masses greater than 10^10 Msun and redshifts
0.025<z<0.05. This release includes new Arecibo observations for 250 galaxies.
We use the full GASS sample to investigate environmental effects on the cold
gas content of massive galaxies at fixed stellar mass. The environment is
characterized in terms of dark matter halo mass, obtained by cross-matching our
sample with the SDSS group catalog of Yang et al. Our analysis provides, for
the first time, clear statistical evidence that massive galaxies located in
halos with masses of 10^13-10^14 Msun have at least 0.4 dex less HI than
objects in lower density environments. The process responsible for the
suppression of gas in group galaxies most likely drives the observed quenching
of the star formation in these systems. Our findings strongly support the
importance of the group environment for galaxy evolution, and have profound
implications for semi-analytic models of galaxy formation, which currently do
not allow for stripping of the cold interstellar medium in galaxy groups.

Cosmology and Nongalactic Astrophysics