The stability of +1 oxidation state among Al,
$\mathrm{Ga}$, In and $\mathrm{Tl}$ increases in the sequence
$\mathrm{Al} < \mathrm{Ga} < \mathrm{In} < \mathrm{Tl}$
This is due to inert pair effect due to which on moving down the group, the stability of +3 oxidation state decreases while stability of +1 oxidation state increases.