  Exclusive heavy-vector-meson photoproduction is a prominent signal in
collider experiments with hadron beams. At the highest photon-hadron collision
energies, this process is considered as a candidate to constrain gluon parton
distribution functions (PDFs) at small longitudinal momentum fractions.
However, in the framework of collinear factorisation, exclusive particle
production is described in terms of generalised parton distributions (GPDs). In
this contribution, we investigate at the leading order in $\alpha_s$ the
connection between GPDs and PDFs. Our main result is a proposal to quantify the
systematic uncertainty inherent to this connection. We put our approach into
context with respect to the Shuavev transform. Our uncertainty estimate can be
straightforwardly adapted to higher fixed orders and small-$x$ resummations.
The question of extrapolating GPDs to vanishing skewness is paramount for the
programme of the Electron Ion Collider (EIC), notably for the extraction of the
radial distributions of partons.
