Electric field strength in helium gamma-mode RF (13.56 MHz) atmospheric pressure glow discharge was measured using Stark polarization spectroscopy. Time-correlated single photon counting was applied to record the temporal development of spectral profile of He I 492.2 nm line with a sub-nanosecond temporal resolution. Electric fields up to 32 kV/cm in the RF sheath, obtained from the fit of forbidden (2 P-1 -4 F-1) and allowed (2 P-1 - 4 D-1) helium lines, are in agreement with the spatially averaged value of 40 kV/cm estimated from homogeneous charge density RF sheath model. The observed rectangular shape of the electric field time development is attributed to increased sheath conductivity by strong electron avalanches occurring in the gamma-mode sheath at high current densities.