We present IRAM-30 m Telescope (CO)-C-12 and (CO)-C-13 observations of a sample of 55 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) in the local universe. This sample is a subset of the Great Observatory All-Sky LIRG Survey (GOALS), for which we use ancillary multi-wavelength data to better understand their interstellar medium and star formation properties. Fifty-three (96%) of the galaxies are detected in (CO)-C-12, and 29 (52%) are also detected in (CO)-C-13 above a 3 sigma level. The median full width at zero intensity (FWZI) velocity of the CO line emission is 661 km s(-1), and similar to 54% of the galaxies show a multi-peak CO profile. Herschel photometric data is used to construct the far-IR spectral energy distribution of each galaxy, which are fit with a modified blackbody model that allows us to derive dust temperatures and masses, and infrared luminosities. We make the assumption that the gas-to-dust mass ratio of (U)LIRGs is comparable to local spiral galaxies with a similar stellar mass (i.e., gas/dust of mergers is comparable to their progenitors) to derive a CO-to-H-2 conversion factor of = 1.8(-0.8)(+1.3) M-circle dot (K km s(-1) pc(2))(-1); such a value is comparable to that derived for (U)LIRGs based on dynamical mass arguments. We derive gas depletion times of 400 600 Myr for the (U)LIRGs, compared to the 1.3 Gyr for local spiral galaxies. Finally, we re-examine the relationship between the (CO)-C-12/(CO)-C-13 ratio and dust temperature, confirming a transition to elevated ratios in warmer systems.