  Long and short gamma-ray bursts are traditionally associated with galactic
environments, where circumburst densities are small or moderate (few to
hundreds of protons per cubic cm). However, both are also expected to occur in
the disks of Active Galactic Nuclei, where the ambient medium density can be
much larger. In this work we study, via semi-analytical methods, the
propagation of the GRB outflow, its interaction with the external material, and
the ensuing prompt radiation. In particular, we focus on the case in which the
external shock develops early in the evolution, at a radius that is smaller
than the internal shock one. We find that bursts in such high density
environments are likely characterized by a single, long emission episode that
is due to the superposition of individual pulses, with a characteristic hard to
soft evolution irrespective of the light curve luminosity. While multi-pulse
light curves are not impossible, they would require the central engine to go
dormant for a long time before re-igniting. In addition, short GRB engines
would produce bursts with prompt duration that would exceed the canonical 2 s
separation threshold and would likely be incorrectly classified as long events,
even though they would not be accompanied by a simultaneous supernova. Finally,
these events have a large dynamical efficiency which would produce a bright
prompt emission followed by a somewhat dim afterglow.
