The Sindarin suffix go- “together” was established early in Tolkien’s life. It was derived from √WO and had a variant gwa- depending on ancient patterns of stress, since ancient wo > wa > gwa when stressed but when unstressed wo > gwo > go (WJ/367-368). The go-variant is the normal form, and gwa- survived only in a few two-syllable words such as gwanon “twin = ✱together-born”.
Conceptual Development: The go-/gwa- variation dates all the way back to the Gnomish Lexicon of the 1910s, but in that document the primitive form was ᴱ✶ŋu̯a which become go- when unstressed (GL/40). The derivation of N. go- from ᴹ√WŌ was introduced in The Etymologies of the 1930s which when stressed produced gwa-, but this gwa-variant was already limited to old formations in the 1930s (Ety/WŌ).
A neologism coined by Paul Strack in 2019 specifically for Eldamo, the Sindarin equivalent of ᴹQ. kumba “bellied”, where the vowel *o* is the result of *a*-affection.