A word appearing as G. gais “steel” in the Gnomish Lexicon where it was cognate to {yakse >>} ᴱQ. yaisa “steel” (GL/37).

Neo-Sindarin: For purposes of Neo-Quenya I use ᴺQ. axë (axi-) “steel” based on words from the 1920s and derived from *gaksē via the Neo-Root ᴺ√GAK. In the phonology of the 1930s, this would produce ᴺS. gaes “steel”; compare to N. taes “nail” < ᴹ✶taksē. Based on this, I use gaes for “steel” in my version of Neo-Sindarin.

There is evidence that the phonological developments in Sindarin of the 1950s and 60s were different, however, given S. ach “neck” < ✶aks (PE17/92). In this later phonology, the result would be *gach. Since I retain words like N. taes, I’m comfortable with words like gaes “steel” as dialectical variants. If you dislike this, you might instead use ᴺS. thornang “steel, (lit.) hard-iron” as a cognate to Q. tornanga “hard-iron”.