The production of prompt photons in proton-proton collisions at the LHC provides a testing ground for perturbative QCD with a hard colourless probe. Since the dominant production mechanism in proton-proton collisions proceeds via the $qg\rightarrow g\gamma$ process, measurements of prompt-photon and photon plus jet production are sensitive at leading order to the gluon density in the proton. This contribution presents the latest ATLAS measurements of the ratio of the cross sections for inclusive isolated-photon production at centre-of-mass energies of 13 and 8 TeV, the measurements of isolated-photon plus jet cross sections at $\sqrt{s}$ = 13 TeV using a dataset with an integrated luminosity of 3.2 fb$^{-1}$ and the differential cross section for isolated-photon production measured at $\sqrt{s}$ = 13 TeV using an integrated luminosity of 36.1 fb$^{-1}$. The measurements are compared to next-to-leading-order QCD as well as to next-to-next-to-leading-order QCD calculations.