Transverse-momentum-dependent gluon distribution functions in a spectator model Alessandro Bacchetta, Francesco Giovanni Celiberto, Marco Radici and Pieter Taels Eur. Phys. J. C, 80 8 (2020) 733 Published online: 12 August 2020 DOI: 10.1140/epjc/s10052-020-8327-6