In Group 13 elements (B, Al, Ga, In, Tl), the stability of the lower oxidation state (+1) increases down the group while the stability of the higher oxidation state (+3) decreases. This is due to the inert pair effect, which is the reluctance of the valence $ns^2$ electrons to participate in bonding because of poor shielding by the intervening $d$ and $f$ electrons. Therefore, the stability of the +1 oxidation state follows the order: $Al^+ < Ga^+ < In^+ < Tl^+$. Hence, $Ga^+$ is more stable than $Al^+$. Note: 'ln+' in the original text is a common typographical error for $In^+$ (Indium ion).