The letter ‘G’ was introduced in the Old Latin period as a variant of ‘C‘ to distinguish voiced /ɡ/ from voiceless /k/. The recorded originator of ‘G’ is freedman Spurius Carvilius Ruga, the first Roman to open a fee-paying school, who taught around 230 BC. At this time, ‘K‘ had fallen out of favor, and ‘C’, which had formerly represented both /ɡ/ and /k/ before open vowels, had come to express /k/ in all environments.
Ruga’s positioning of ‘G’ shows that alphabetic order related to the letters’ values as Greek numerals was a concern even in the 3rd century BC. According to some records, the original seventh letter, ‘Z’, had been purged from the Latin alphabet somewhat earlier in the 3rd century BC by the Roman censor Appius Claudius, who found it distasteful and foreign. Sampson (1985) suggests that: “Evidently the order of the alphabet was felt to be such a concrete thing that a new letter could be added in the middle only if a ‘space’ was created by the dropping of an old letter.”
Hempl (1899) proposes that there never was such a “space” in the alphabet and that in fact ‘G’ was a direct descendant of zeta. Zeta took shapes like ⊏ in some of the Old Italic scripts; the development of the monumental form ‘G’ from this shape would be exactly parallel to the development of ‘C’ from gamma. He suggests that the pronunciation /k/ > /ɡ/ was due to contamination from the also similar-looking ‘K’.
Eventually, both velar consonants /k/ and /ɡ/ developed palatalized allophones before front vowels; consequently in today’s Romance languages, ⟨c⟩ and ⟨g⟩ have different sound values depending on context (known as hard and soft C and hard and soft G). Because of French influence, English orthography shares this feature.