  We present the first measurements of [Fe/H] and [$\alpha$/Fe] abundances,
obtained using spectral synthesis modeling, for red giant branch stars in M31's
giant stellar stream. The spectroscopic observations, obtained at a projected
distance of 17 kpc from M31's center, yielded 61 stars with [Fe/H]
measurements, including 21 stars with [$\alpha$/Fe] measurements, from 112
targets identified as M31 stars. The [Fe/H] measurements confirm the
expectation from photometric metallicity estimates that stars in this region of
M31's halo are relatively metal-rich compared to stars in the MW's inner halo:
more than half the stars in the field, including those not associated with
kinematically identified substructure, have [Fe/H] abundances $> -1.0$. The
stars in this field are $\alpha$-enhanced at lower metallicities, while
[$\alpha$/Fe] decreases with increasing [Fe/H] above metallicities of [Fe/H]
$\gtrsim -0.9$. Three kinematical components have been previously identified in
this field: the giant stellar stream, a second kinematically cold feature of
unknown origin, and M31's kinematically hot halo. We compare probabilistic
[Fe/H] and [$\alpha$/Fe] distribution functions for each of the components. The
giant stellar stream and the second kinematically cold feature have very
similar abundance distributions, while the halo component is more metal-poor.
Although the current sample sizes are small, a comparison of the abundances of
stars in the giant stellar stream field with abundances of M31 halo and dSph
stars from the literature indicate that the progenitor of the stream was likely
more massive, and experienced a higher efficiency of star formation, than M31's
existing dSphs or the dEs NGC147 and NGC185.
