  We compute $t_0$, $w_0$ and the topological susceptibility, defined at finite
gradient flow time for two-flavour QCD. The use of three lattice spacings and
pion masses between 192 and 500 MeV together with a careful error analysis
allow to approach the continuum limit of the two-flavour theory despite
significant auto-correlations. A comparison to $N_f=0$ results shows the size
of sea quark effects in $t_0^2\chi$, with $\chi$ the topological
susceptibility, and low energy observables such as $t_0/w_0^2$ and $t_0/r_0^2$.
