  Using the Gemini Near-InfraRed Spectrograph (GNIRS), we have completed a
near-infrared spectroscopic survey for K-bright galaxies at z~2.3, selected
from the MUSYC survey. We derived spectroscopic redshifts from emission lines
or from continuum features and shapes for all 36 observed galaxies. The
continuum redshifts are driven by the Balmer/4000 Angstrom break, and have an
uncertainty in dz/(1+z) of <0.019. We use this unique sample to determine, for
the first time, how accurately redshifts and other properties of massive
high-redshift galaxies can be determined from broadband photometric data alone.
We find that the photometric redshifts of the galaxies in our sample have a
systematic error of 0.08 and a random error of 0.13 in dz/(1+z). The systematic
error can be reduced by using optimal templates and deep photometry; the random
error, however, will be hard to reduce below 5%. The spectra lead to
significantly improved constraints for stellar population parameters. For most
quantities this improvement is about equally driven by the higher spectral
resolution and by the much reduced redshift uncertainty. Properties such as the
age, A_V, current star formation rate, and the star formation history are
generally very poorly constrained with broadband data alone. Interestingly
stellar masses and mass-to-light ratios are among the most stable parameters
from broadband data. Nevertheless, photometric studies may overestimate the
number of massive galaxies at 2<z<3, and thus underestimate the evolution of
the stellar mass density. Finally,the spectroscopy supports our previous
finding that red galaxies dominate the high-mass end of the galaxy population
at z=2-3.
