  The idea that GRBs originate from uniform jets has been used to explain
numerous observations of breaks in the GRB afterglow lightcurves. We explore
the possibility that GRBs instead originate from a structured jet that may be
quasi-universal, where the variation in the observed properties of GRBs is due
to the variation in the observer viewing angle. We test how various models
reproduce the jet data of Bloom, Frail, & Kulkarni (2003), which show a
negative correlation between the isotropic energy output and the inferred jet
opening angle (in a uniform jet configuration). We find, consistent with
previous studies, that a power-law structure for the jet energy as a function
of angle gives a good description. However, a Gaussian jet structure can also
reproduce the data well, particularly if the parameters of the Gaussian are
allowed some scatter. We place limits on the scatter of the parameters in both
the Gaussian and power-law models needed to reproduce the data, and discuss how
future observations will better distinguish between these models for the GRB
jet structure. In particular, the Gaussian model predicts a turnover at small
opening angles and in some cases a sharp cutoff at large angles, the former of
which may already have been observed. We also discuss the predictions each
model makes for the observed luminosity function of GRBs and compare these
predictions with the existing data.
