We investigate a novel mechanism of photon energy gain in an optically thick relativistic plasma with velocity shear. It is a Fermi-like process where the photons repeatedly scatter within the regions of different Lorentz factors, thereby gaining energy on average. The resulting spectra have a power-law shape at high energies. Thus, this mechanism is an alternative to the classical emission from power-law accelerated particles that can generate power-law spectra in sources such as Gamma-ray bursts (GRBs) and Active Galactic Nuclei (AGNs). Using both numerical simulations and theoretical calculations, we compute the expected spectra for parameters characterizing GRBs and recover the range of their observed photon indices at high energies.