  Gradual shear acceleration of energetic particles in gamma-ray burst (GRB)
jets is considered. Special emphasis is given to the analysis of universal
structured jets, and characteristic acceleration timescales are determined for
a power-law and a Gaussian evolution of the bulk flow Lorentz factor $\gamma_b$
with angle $\phi$ from the jet axis. The results suggest that local power-law
particle distributions may be generated and that higher energy particles are
generally concentrated closer to the jet axis. Taking several constraints into
account we show that efficient electron acceleration in gradual shear flows,
with maximum particle energy successively decreasing with time, may be possible
on scales larger than $r \sim 10^{15}$ cm, provided the jet magnetic field
becomes sufficiently weak and/or decreases rapidly enough with distance, while
efficient acceleration of protons to ultra-high energies $> 10^{20}$ eV may be
possible under a wide range of conditions.
