  We present a statistically robust mass-metallicity (M-Z) relation for
long-duration gamma-ray burst (LGRB) host galaxies at z < 1. By comparing the
LGRB host M-Z relation to samples representative of the general star-forming
galaxy population, we conclude that LGRBs occur in host galaxies with lower
metallicities than the general population, and that this trend extends to z ~
1, with an average offset of -0.42 +/- 0.18 from the M-Z relation for
star-forming galaxies. Our sample in this work includes new spectroscopic data
for 6 LGRB host galaxies obtained at the Keck and Magellan telescopes, as well
as 2 new host galaxies from the literature. Combined with data from our
previous work, this yields a total sample of 6 LGRB host galaxies at z < 0.3
and 10 host galaxies at 0.3 < z < 1. We have determined a number of
interstellar medium properties for our host galaxies using optical
emission-line diagnostics, including metallicity, ionization parameter, young
stellar population age, and star formation rate. Across our full sample of 16
LGRB hosts we find an average metallicity of log(O/H) + 12 = 8.4 +/- 0.3.
Notably, we also measure a comparatively high metallicity of log(O/H) + 12 =
8.83 +/- 0.1 for the z = 0.296 host galaxy of GRB 050826. We also determine
stellar masses (M*) for our LGRB host galaxy sample, finding a mean stellar
mass of log(M*/Msun) = 9.25 (+0.19,-0.23).
