Tantrasara (Krishnananda) 2.p609

2.p609

atha vakṣye gaṇapatermantrānsarvārthasiddhidān .
yān jñātvā mānavā nityaṃ sādhayanti manorathān .. 59 ..
pañcāntakaṃ śaśidharaṃ bījaṃ gaṇapaterviduḥ .
pañcāntakaṃ gakāraḥ .. 60 ..

Now I shall speak of the mantras of Gaṇapati, which grant the achievement of all goals, / knowing which men perpetually accomplish their wishes. // 59
They know the bīja of Gaṇapati to be that which ends with the fifth [vowel] and bears the moon.
“That which ends with the fifth” is the letter “ga”. // 60