Ground state term symbol for Cerium (Ce)
Iván, you are right on your determination using Hund's rules. However, for heavy elements, with strong spin-orbit coupling, the LS coupling assumed by Hund's rules may not apply. In this case, the j-j coupling may give better agreement with experiment. 

There are extensions of Hund's rules for j-j coupling [1]. 

If you check NIST Atomic Spectra Database Levels Data, you will see that the wavefunction for the ground state of the Ce atom is 55% $^1\text{G}^\circ$ and 29% $^3\text{H}^\circ$. This mixture between singlet and triplet is due to the spin-orbit coupling. 

[1] T. E. H. Walker, J. T. Waber, Modified Hund’s Rule for jj Coupling. Physical Review A. 7, 1218–1224 (1973). https://dx.doi.org/10.1103/PhysRevA.7.1218