  We report measurements of the carbon monoxide ground state rotational
transition (12C16O J = 1--0) with the Zpectrometer ultra-wideband spectrometer
on the 100-m diameter Green Bank Telescope. The sample comprises 11 galaxies
with redshifts between z = 2.1 and 3.5 from a total sample of 24 targets
identified by Herschel-ATLAS photometric colors from the SPIRE instrument. Nine
of the CO measurements are new redshift determinations, substantially adding to
the number of detections of galaxies with rest-frame peak submillimeter
emission near 100um. The CO detections confirm the existence of massive gas
reservoirs within these luminous dusty star-forming galaxies (DSFGs). The CO
redshift distribution of the 350um-selected galaxies is strikingly similar to
the optical redshifts of 850um-selected submillimeter galaxies (SMGs) in 2.1 <
z < 3.5. Spectroscopic redshifts break a temperature-redshift degeneracy;
optically thin dust models fit to the far-infrared photometry indicate
characteristic dust temperatures near 34 K for most of the galaxies we detect
in CO. Detections of two warmer galaxies and statistically significant
nondetections hint at warmer or molecule-poor DSFGs with redshifts difficult
determine from from Herschel-SPIRE photometric colors alone. Many of the
galaxies identified by H-ATLAS photometry are expected to be amplified by
foreground gravitational lenses. Analysis of CO linewidths and luminosities
provides a method for finding approximate gravitational lens magnifications mu
from spectroscopic data alone, yielding mu ~ 3--20. Corrected for
magnification, most galaxy luminosities are consistent with an ultra-luminous
infrared galaxy (ULIRG) classification, but three are candidate hyper-LIRGs
with luminosities greater than 10^13 L_sun.
