Phenomenology of gluon TMDs from $\eta_{b,c}$ production

We present the potential of $\eta_{b,c}$ production in proton collisions to access the gluon transverse-momentum dependent parton distributions (TMDs). In particular, we explore the phenomenology of the unpolarized and linearly-polarized gluon TMDs in unpolarized collisions for different kinematic settings, and the potential of a fixed-target experiment at the LHC.