  Various studies firmly establish the fact that gamma-ray observations can act
as a unique probe to detect the possible cosmic ray (CR) sources, study the CR
density distribution and explore the average properties of interstellar medium
(ISM) such as the gas density profile of ISM. We use the DRAGON code to study
different propagation models by incorporating realistic source distribution,
Galactic magnetic field (GMF) and gas density profile, and finally obtain the
proton distribution (both spatial and energy) in the Galaxy by fitting the
locally observed CR spectra. The uncertainties in the model parameters used to
study the CR propagation are also shown here. Our obtained proton distribution
is then used to calculate the diffuse gamma-ray flux produced by proton-proton
interactions in the Galactic halo. We compare our diffuse gamma-ray fluxes with
the previous results and the isotropic gamma-ray background (IGRB) at sub -TeV
energy regime measured by \textit{Fermi}-LAT. It is found to be much less than
IGRB, which suggests IGRB is mostly of extragalactic origin. Finally, we use
our obtained proton distribution to calculate gamma-ray fluxes from individual
Galactic Molecular Clouds (GMCs) and those fluxes are compared with the
\textit{Fermi}-LAT observations of GMCs.
