  Gamma-ray bursts (GRBs) are known to have the most relativistic jets, with
initial Lorentz factors in the order of a few hundreds. Many GRBs display an
early X-ray light-curve plateau, which was not theoretically expected and
therefore puzzled the community for many years. Here, we show that this
observed signal is naturally obtained within the classical GRB "fireball"
model, provided that the initial Lorentz factor is rather a few tens, and the
expansion occurs into a medium-low density "wind". The range of Lorentz factors
in GRB jets is thus much wider than previously thought and bridges an
observational gap between mildly relativistic jets inferred in active galactic
nuclei, to highly relativistic jets deduced in few extreme GRBs. Furthermore,
long GRB progenitors are either not Wolf-Rayet stars, or the wind properties
during the final stellar evolution phase are different than at earlier times.
We discuss several testable predictions of this model.
