Question

why gallium has less atomic radii than aluminium ????and p block atomic radii increases as we go down the group

Answer 1

Even though Gallium lies below Aluminum, the atomic size of Gallium is smaller than Aluminum because of Shielding Effect. ... In case of Gallium the outermost electrons are poorly shielded by d electrons increasing nuclear attraction of the outer electrons, which results the smaller radius of Gallium.

The atomic radius of the of p-block elements generally decreases on moving across a period from left to right in the periodic table. It is because the addition of electrons takes place in the same valence shell and are subjected to an increased pull of the nuclear charge at each step.