Shielding effect of d electrons
When we are talking about zinc... We have to think about period trends. So in periods along moving from left to right covalent radius decreases. But in case of transition metal..

Two effects works first is $Z_{eff}$. Another effect is electronic repulsion.At the starting the no of electron in $d-$ orbital are less so $Z_{eff}$ is dominant over electronic repulsion but as we are moving alosng d-orbital electronic effect become dominant factor over $Z_{eff}$, So in the middle redius become almost constant and at the end it increases.

When we are talking about gallium and Aluminium radius comparision thats mean we are talking about group trends ,So in group trend abnormal size is due to poor shielding effect and relativistic effect. Normal trend on going down the size increases from aluminium to gallium size decreases because here a poor shielding of 3-d orbital present so $Z_{eff}$ penetarate and valence shell contract so size decreases.