The pronunciation of the stops b, d, g / p, t, k is—it seems—unstable. In German, the voiced stops are unvoiced at the end of words, such as in lieb mir, das Lied, Guten Tag (/liːp mi:r, das liːt, guːt(ə)n taːk/); in Korean, they are unvoiced word-initially, such as in 보 (bo: /po/), 단군 (Dan-gun: /tangun/), 관 (gwan: /kwan/); in Norwegian, Swedish, Danish and English, a -p- is voiced e.g. post initial s before a vowel, such as in spade (no: /²sbaːde/; en: /sbeɪd/).¹ Without going to deeply into linguistics territory: It is a well-established phenomenon.
Latin, it would seem, has the same phenomenon represented from older times, with the etruscan C serving both as /k/ and /g/; hence ‘C.’ is short for Gaius. Gian Biagio Conte writes that
What is left shows the existence of a melting pot of peoples and languages. Only gradually does the use of Latin and the Latin alphabet assert itself. The Latin alphabet itself, in fact, gives clear testimony of the situation in early Rome. In substance, it is derived from a particular West Greek alphabet, the one used in the powerful Campanian city of Cumae, but it is also somewhat influenced by Etruria. (This explains, for example, how the letter C serves as the abbreviation for the name Gaius: Etruscan had a single sign for the two velar consonants, voiceless and voiced.
I have read this and similar explanations many places, but it seems no-one entertains the notion that the early speakers might have had a devoicing of initial /g/ to /k/. My question, thus, is did the early Latin speakers devoice initial stops?
As a final note, it is probably worthy of note to mention the perhaps unstable spelling of words with an initial /k/: If borrowed from Greek and followed by α, it might get the spelling ka, such as in kalendae (e.g. ‘Martis Romani festae uenere kalendae | —exoriens nostris hic fuit annus auis—’ Tib.3.1); if followed by consonants or the vowels a, e, i, it is always c; but if followed by v—pronounced /uː/, /ʊ/ or /w/—it is spelt either q or c. This article by Sturtevant provides a useful comment on this.
¹ Though this might be contended. Is it merely an unaspiration?