  Direct photon production in hadronic collisions provides a handle on the
gluon PDF by means of the QCD Compton scattering process. In this work we
revisit the impact of direct photon production on a global PDF analysis,
motivated by the recent availability of the next-to-next-to-leading (NNLO)
calculation for this process. We demonstrate that the inclusion of NNLO QCD and
leading-logarithmic electroweak corrections leads to a good quantitative
agreement with the ATLAS measurements at 8 TeV and 13 TeV, except for the most
forward rapidity region in the former case. By including the ATLAS 8 TeV direct
photon production data in the NNPDF3.1 NNLO global analysis, we assess its
impact on the medium-x gluon. We also study the constraining power of the
direct photon production measurements on PDF fits based on different datasets,
in particular on the NNPDF3.1 no-LHC and collider-only fits. We also present
updated NNLO theoretical predictions for direct photon production at 13 TeV
that include the constraints from the 8 TeV measurements.
