1Ionisation enthalpy generally decreases down the group. But in this case there is a slight increase in the case of Ga and Th. This is because in case of Ga , 3d subshell is present and in case of Th, 4f subshell is present.
3d and 4f provide poor shielding, so they can't account for the increase in nuclear charge ( which has increased more due to presence of 3d elements and 4f elements resp ).
Therefore , I.E. is more in case of these two..
