Well, the trend for the ionisation enthalpy is :
First, the ionisation enthalpy of Ga is more than that of Aluminium, regardless of the fact that Aluminium is above Gallium, because of the Fully Filled inner d-Subshell in case of Gallium. Since there are 10 d-electrons added in case of Gallium, thus the effect of increased Atomic Radius is offset by the extremely large increase in the Nuclear Charge, due to the poor shielding by the d-electrons, as a result of which the effective Nuclear Charge increases in case of Gallium, as compared to Aluminium, and hence the Ionisation Energy of Gallium is more than that of Aluminium.
Secondly, the Ionisation Energy of Thallium is more than that of indium because of Lanthanide Contraction, as per which, as the 4f subshell is completely filled with 14 Electrons, therefore there exists very high effective Nuclear Charge, due to very poor shielding of the 4f electrons. Hence there is a very large increment in the IP of Thallium. This makes the ionisation enthalpy of Thallium to be even more than Aluminium and Gallium.
Do let me know if u have any doubts.