  The Zwicky Transient Facility recently announced the detection of an optical
transient AT2020blt at redshift $z=2.9$, consistent with the afterglow of an
on-axis gamma-ray burst. However, no prompt emission was observed. We analyse
AT2020blt with detailed models, showing the data are best explained as the
afterglow of an on-axis long gamma-ray burst, ruling out other hypotheses such
as a cocoon and a low-Lorentz factor jet. We search \textit{Fermi} data for
prompt emission, setting deeper upper limits on the prompt emission than in the
original detection paper. Together with KONUS-\textit{Wind} observations, we
show that the gamma-ray efficiency of AT2020blt is $\lesssim 0.3 - 4.5\%$. We
speculate that AT2020blt and AT2021any belong to the low-efficiency tail of
long gamma-ray burst distributions that are beginning to be readily observed
due to the capabilities of new observatories like the Zwicky Transient
Facility.
