An adjective meaning “bright” derived from the root ᴹ√GAL, the basis of the river name Gelion (Ety/GYEL). There isn’t enough information to deduce its primitive form, but Helge Fauskanger suggested ✶galjānā* (AL-Ilkorin/gelion), which seems reasonably plausible.
Doriathrin
ar-
prefix. outside, beside
gelion
adjective. bright
An Ilkorin prefix meaning “outside, beside”, the same as in Quenya and Noldorin (Ety/AR²).