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).
Thus F came to be the representative of the unvoiced labiodental spirant instead of that for the bilabial voiced spirant.
The differentiation of the Roman alphabet from the Greek is brought about (a) by utilizing the digamma for the unvoiced labio dental spirant F; (b) by dropping out the aspirates 0, (I), in the Chalcidian alphabet, whence the Roman is derived) from the alphabet proper and employing them on l as numerals, 0 'y' ?
There is some evidence which seems to point to a pronunciation of the voiced mutes which, like the South German pronunciation of g, d, b, but slightly differentiated them from the unvoiced mutes, so that confusion might easily arise.
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.