There are two characters with this name in Tolkien’s writings: a Noldorin Elf who met Frodo as he was leaving the Shire with the surname Inglorion (LotR/80), and a man in the band of outlaws led by Barahir (S/155). The name appears to be a combination of gil “star” and this suffix -dor “lord”.
Conceptual Development: The outlaw N. Gildor first appeared in Silmarillion drafts from the 1930s (LR/133), while Inglorion first appeared in Lord of the Rings drafts from the 1940s (RS/60).
gîl (“star, bright spark”) + taur (“noble, king, lord”) The second element is only used poetically, as -dor often found in names [Etym. TĀ-].