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).
At the end of words the voiced sound is often written with -s, the unvoiced with -ss as in his and hiss.
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 medieval and modern Greek, however, this has become the unvoiced sound represented in English by th in thin, thick, pith.
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.