Here's *THE LATIN* reference, the Lewis and Short Lexicon [with minor correction from my hardcopy]:
http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3D%2319181
G , g , indecl. n. or (on account of littera) f., had originally no place in the Latin alphabet: both the sharp and the flat guttural mutes, our k and g sounds, being represented by C; hence on the Columna Rostrata LECIONES, MACISTRATOS, EXFOCIONT, (pu)CNANDOD, PVCN(ad), CARTACINIENSIS, for legiones, etc.; hence, too, the archaic form ACETARE for agitare (v. Paul. ex Fest. p. 23 Müll. N. cr.), and the still common abbreviation of the names Gaius and Gneus in C and Cn.At a later period (acc. to Plut. Qu. Rom. p. 277 D and 278 E, by means of a freedman of Spurius Carvilius Ruga, about the beginning of the second Punic war) a slight graphic alteration was made in the C, which introduced into the Roman orthography the letter G (on the old monuments C); thus we have in the S. C. de Bacchanal.: MAGISTER, MAGISTRATVM, FIGIER, GNOSCIER, AGRO; on the other hand, the orthography GNAIVOD PATRE PROGNATVS on the first Epitaph of the Scipios, which dates before that time, indicates either incorrectness in the copying or a later erection of the monument.