  The GALPROP model for cosmic-ray propagation is able to make explicit
predictions for the distribution of galactic diffuse gamma-rays. We compare
different propagation models with gamma-ray spectra measured by EGRET for
various regions of the sky. This allows sensitive tests of alternative
explanations for the apparent excess emission observed at GeV gamma-rays. We
find that a population of hard-spectrum gamma-ray sources cannot be solely
responsible for the excess since it also appears at high latitudes; on the
other hand a hard cosmic-ray electron spectrum cannot explain the gamma-ray
excess in the inner Galaxy. By normalizing the cosmic ray spectra within
reasonable bounds under preservation of their shape we are able to obtain our
best prediction of the Galactic component of diffuse gamma rays, and show that
away from the Galactic plane it gives an accurate prediction of the observed
gamma-ray intensities. On this basis we reevaluate the extragalactic gamma-ray
background. We find that for some energies previous work underestimated the
Galactic contribution and hence overestimated the background. The new EGRB
spectrum shows a positive curvature similar to that expected for models of the
extragalactic gamma-ray emission based on contributions from unresolved
blazars.
