How to determine the capacitance from the capacitor charging current
If you integrate the current over the time, you get \$[As]\$. This is the charge \$Q\$. The charge of a cap is given by \$Q_C=C\cdot U\$.

Have a look at the energy of a cap, it defined by \$E_{C}=\frac{1}{2}\cdot C\cdot U^2\$ and the relation to the current is \$i_C(t) = C\cdot\frac{du_C(t)}{dt} \$.

With this equations you can see, that only looking at time and current can not load to the capacitance, you also have to take the voltage curve in consideration.
VG1 might not be correct, because there is voltage drop above R1.