Question

why ionisation enthalpy of ga is higher than Al?

Answer 1

Insufficient shielding by the 3d subshell. After Calcium, the 3d subshell fills up with 10 electrons from Scandium to Zinc. The number of protons go up as well. All the ten electrons are at the same mean distance from the nucleus which  means that when the 4p subshell starts filling up from Ga, the lone electron in Ga's 4p subshell is closer to the nucleus than it would have been if all the previous 10 electrons had not gone into one subshell.

Being closer to the nucleus than a "linear" extension of Al's config means that more energy has to be spent to remove the outermost electron in Ga, and hence it's ionization energy is higher than Al