The word is deduced from its mutated form, but it is worth mentioning that a stem GALÁN "bright", with glan "daylight" (and later "clear") as derivative, is listed in the Etymologies (not included in the published text, but see VT/45:13). Most of the words meaning "white" in the Indo-Eureopean languages come from the original notion of "brightness", e.g. Greek leukós "white" is cognate with Latin lucere "to shine", lux "light". This association of sense is also found in Gnomish, PE/11:39 (glan "clean, pure", from "bright" originally) and in Early Noldorin (PE/13:144, glann "clean"). The similarity with Welsh glan (where the vowel, incidentally, is also long, though this is concealed by Welsh orthographic convention) is also striking
Sindarin
gelion
place name. Gelion
glân
adjective. bright, shining white
gail
bright
gail (light), lenited ngail; no distinct pl. form (VT45:18). The adj. calen etymologically means "bright", but is used = "green" (q.v.).
gail
bright
(light), lenited ngail; no distinct pl. form (VT45:18). The adj. calen etymologically means "bright", but is used = "green" (q.v.).
gîl
bright spark
(i ngîl = i ñîl, construct gil) (star, silver glint), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. *giliath** (RGEO, MR:388)*
A great river in eastern Beleriand (S/54). The meaning of the name is unclear. Given the river’s location in Ossiriand, this name might be Nandorin instead.
Conceptual Development: The name N. Gelion first appeared in Silmarillion drafts from the early 1930s as a revision of earlier Flend (SM/135). In The Etymologies, Tolkien said the name was based on the Ilkorin word gelion “bright” (Ety/GYEL). In later notes, Tolkien considered renaming the river to Gelduin >> Gevilon >> Gevelon or Gevolon, the last of these an adaptation of Khuzdul Gabilân “Great River” (WJ/336; NM/311). He did not apply this change in the Silmarillion revisions from the 1950s-60s, however.