  Gamma-ray bursts (GRB) are generally believed to be efficient particle
accelerators. In the presence of energetic protons in a GRB jet, interactions
between these protons and intense radiation field of the GRB are supposed to
induce electromagnetic cascade. Electrons/positrons generated in the cascade
will produce an additional spectrum of robust feature, which is in the form of
a power-law distribution up to GeV regime with an index of $\lesssim 2$. We
suggest that measurements of Fermi-LAT at GeV band can provide independent
constraints on the key GRB model parameters such as the dissipation radius, the
jet's bulk Lorentz factor, and the baryon loading factor. Taking GRB 221009A,
the brightest GRB ever detected, as an example, we show that the constraints
from GeV gamma-ray emission may be more stringent than that from the neutrino
observation, providing us a deep insight into the origin of GRBs.
