  We analytically examine the capabilities of rotation-powered pulsars as the
sources of gamma-rays and show that their phase-averaged gamma-ray flux is
proportional to the product of the spin-down flux and the gap trans-field
thickness cubed irrespective of the emission models. Applying the scheme to the
Crab pulsar, we demonstrate that the outer-gap model reproduces the observed
GeV fluxes and that the slot-gap model reproduces at most twenty per cent of
the observed fluxes because of the small trans-field thickness. An implication
on the relationship between the gamma-ray and the spin-down fluxes is
discussed.
