A term the Sindar used for the Elves who left Beleriand for Aman, derived from the same root as Q. vanwa: √WĀ/AWA (WJ/366, 378). Another variation was Gwanwel (WJ/378), perhaps incorporating †Ell “Elf”.
Conceptual Development: Notes on Names (NN) from 1957 had a similar form Gwenedhel, with deleted variants {gwenieðel} and {gweineðel} ultimately revised to Caleðel “✱Light Elf” (PE17/140).
A word for “departed” in the Quendi and Eldar (Q&E) essay from 1959-60 (WJ/378), clearly derived from the root √WĀ “away”. It might be related to the verb gwae- from Definitive Linguistic Notes (DLN) of 1959 (PE17/148). That verb had an irregular passive participle gwawn or gwanu. The word gwanwen may be a modification of (archaic?) gwanu from DLN to more strongly resemble other passive participles like govannen (LotR/209) or onnen (WJ/387). Alternately, it could be a simple adjective, the Sindarin equivalent of Q. vanwa.
Neo-Sindarin: I would assume that, like its Quenya equivalent, gwanwen has the added connotation of “✱gone, lost [to time], past” for purposes of Neo-Sindarin.
Conceptual Development: The Gnomish Lexicon of the 1910s had G. haim or haithin “gone, departed, lost”, the latter based on the verb G. haitha- “hie, go, fare, walk” (GL/47).