While we await one of our Latin scholars (O Faldage, where art thou?) to appear, let me throw in my guess:

To rephrase what Jo said: Just as the C (cf. celtic) was pronounced K in Latin, as far as we know, so was the G a hard sound and not soft (cf. Regina - hard G sound). Today we pronounce the singular and plural as we do because of English conventions governing the G sound in the two pre-vowel environments.