The symbol G was a new coinage in the 3rd century B.C. The pronunciation of C throughout the period of classical Latin was that of an unvoiced guttural stop (k).
The sound was that of the unvoiced dental stop. The English t, however, is not dental but alveolar, being pronounced, as d also, not by putting the tongue against the teeth but against their sockets.
In English th represents both the unvoiced sound J as in thin, &c., and the voiced sound 5, which is found initially only in pronominal words like this, that, there, then, those, is commonest medially as in father, bother, smother, either, and is found also finally in words like with (the preposition), both.
Early English used ]' and S indiscriminately for both voiced and unvoiced sounds, in Middle English S disappeared and p was gradually assimilated in form to y, which is often found for it in early printing.
Thus F came to be the representative of the unvoiced labiodental spirant instead of that for the bilabial voiced spirant.