For the Ga 3d level, l =2. Thus, spin orbit splitting produces j=3/2 , 5/2. The ratio of the degeneracies of these levels, the branching ratio is : (2 x 3/2) +1: (2 x 5/2) + 1 = 2 : 3.
An s electron has 0 angular momentum and thus spin orbit splitting does not occur - a singlet peak is observed.