gām is the substitute of iṅ when liṭ (Perfect Tense) follows.,
The substitute being like its original (I. 1. 68.) गाङ् would have been Atmanepadi, even without the indicatory, ङ्. Why has then this ङ् been added to it? The object is that in the sutra (1. 2. 1.) गाङ् there should mean this substitute गाङ् and not the verb 'to go.'
Thus we get अधिजगे 'he studied'. अधिजगाते, अधिजगिरे. See 6.4.64 and 3.4.8.,