@meeper According to the 'pedia:
>The evolution of the Latin alphabet's G can be traced back to the Latin alphabet's predecessor, the Greek alphabet. The voiced velar stop was represented by the third letter of the Greek alphabet, gamma (Γ), which was later adopted by the Etruscan language. Latin then borrowed this "rounded form" of gamma, C, to represent the same sound in words such as recei, which was likely an early dative form of rex, meaning "king", as found in an "early Latin inscription." Over time, however, the letter C shifted to represent the voiceless velar stop, leading to the displacement of the letter K. Scholars believe that this change can be attributed to the influence of the Etruscan language on Latin.
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>Afterwards, the letter 'G' was introduced in the Old Latin period as a variant of 'C' to distinguish voiced /ɡ/ from voiceless /k/, and G was used to represent a voiced velar stop from this point on and C "stood for the unvoiced velar only".
So originally they did use K, from Etruscan influence it was entirely displaced by C for both velar sounds, and then they invented G.