No, "age" is not only a preexisting word, it's one that's common as dirt.
Having the rust port be "rage" only compounds the problem.
The author's rather Italian-looking name tends to suggest that he wouldn't view a hard G as a possibility for the word "age" either (and the pronunciation link specifically goes to a synthetic Italian pronunciation of the word "aghe"...), so something else is going on.
I don't understand how the author wants to pronounce it. "ag-ay"? It's very confusing that it's an extremely common word which this program decides to pronounce differently.
> I don't understand how the author wants to pronounce it. "ag-ay"?
More or less. In the technical specifics, the English FACE vowel is a diphthong ending at a pretty high point ( https://en.wikipedia.org/wiki/Vowel#Height ), whereas the indicated pronunciation is a monophthong that should be a bit lower.
You might think of the English vowels in FACE / PRICE / CHOICE / GOAT / MOUTH as being "closed off" by a rapid upward tongue movement towards the end of their pronunciation; what the author is asking for here is a version of "FACE" that doesn't have that closure. Or, alternatively, a version of DRESS [which is already a monophthong] that uses a bit higher of a tongue position.
https://en.wiktionary.org/wiki/git#Etymology_1
https://en.wiktionary.org/wiki/git#Etymology_2
It'd be like naming your software fukr and then insisting "no no no, the R is pronounced 'are', not 'er'."