[thaumasiotes]

As J is just a fancy I, and W is a fancy V. It's a pretty standard way to develop new letters. G is literally a fancy C.

Incidentally, there's a reason the IPA for the sound is /tʃ/. It's two sounds pronounced together, in a manner essentially identical to the /kt/ at the end of the word "act", or the /ts/ at the end of the word "cats".

The parallel is not perfect - in particular, affricates are generally understood as being a single sound, while coarticulated stops are generally thought of as being two sounds in sequence, even where they show all the phonological independence you'd expect of a single phoneme. (Compare e.g. Greek "Ctesias".)