Multi-TeV cosmic rays (CRs) form a "sea" of particles within the Galactic plane, resulting in a diffuse gamma-ray emission visible across a broad energy range. The Fermi Large Area Telescope has measured these gamma-ray emissions at GeV energies with high statistics, and recent results from H.E.S.S., HAWC, LHAASO, and the Tibet Air-shower Array show that the diffuse gamma-ray emission extends into the TeV and PeV energy regimes. The emissions observed at GeV and TeV energies are connected by the common origin of the CR particles injected by the sources; however, the CR sources, their energy dependence, and the relative composition of the CRs, are not well understood.

In this contribution we use the 3D simulation software GALPROP to model CR diffusion across the Galaxy. Using a grid of steady-state models we compute the first quantitative measure of the variation in the diffuse gamma-ray flux that arises from uncertainties in the CR source positions, the interstellar radiation field, and the Galactic magnetic field. We provide the first longitudinal profiles of the modelled gamma-ray emission up to 100 TeV from GALPROP along the Galactic plane and compare the model predictions to the H.E.S.S. Galactic plane survey (HGPS) after carefully subtracting emission from catalogued TeV gamma-ray sources. We found that the GALPROP model predictions agree with the lower estimates for the HGPS source-subtracted diffuse flux. Our results show that the next generation Cherenkov telescope array (CTA) should be able to observe the diffuse emission if appropriate measures are taken to subtract background emission.