The substitution of ava for the o of go is compulsory when a vowel to be found in the word indra follows it.,
Thus गवेन्द्रः ॥ So also गवन्द्र यज्ञः ॥ The word नित्य is not found in some texts or in Mahabhashya.,
इन्द्रे च नित्यम्
Adhyāya 6 · Pāda 1 · Rule 124