  The small-$x$ gluon in global fits of parton distributions is affected by
large uncertainties from the lack of direct experimental constraints. In this
work we provide a precision determination of the small-$x$ gluon from the
exploitation of forward charm production data provided by LHCb for three
different centre-of-mass (CoM) energies: 5~TeV, 7~TeV and 13~TeV. The LHCb
measurements are included in the PDF fit by means of normalized distributions
and cross-section ratios between data taken at different CoM values, $R_{13/7}$
and $R_{13/5}$. We demonstrate that forward charm production leads to a
reduction of the PDF uncertainties of the gluon down to $x\simeq 10^{-6}$ by up
to an order of magnitude, with implications for high-energy colliders, cosmic
ray physics and neutrino astronomy.
