Galdor's date of birth is nowhere recorded, but there are some clues available to us. We know from The Silmarillion that his father Hador was sixty-six years old in I 455 (the date of his death in the Dagor Bragollach). We also know from the Narn i Hîn Húrin in Unfinished Tales that Galdor's son Húrin was 'new-come to manhood' when Galdor himself fell seven years later. These facts favour a date of birth somewhere between I 410 and I 420.

According to Christopher Tolkien in The Etymologies (in The History of Middle-earth volume V), there are two possible Elvish roots that might account for Galdor's name: gala-, meaning to thrive or prosper, or the better known galad-, 'tree'. The '-dor' element of his name seems to mean 'lord'. Galdor's name, then, probably means either 'prosperous lord' or 'tree lord'. Given that his father's House was at the height of its power when he was named, and the country Hador ruled was more or less tree-less, the former seems rather more likely.

Galdor's mother Gildis is not mentioned in any canonical text, but she does appear in a late genealogy reproduced by Christopher Tolkien in volume XI of The History of Middle-earth.

Family Tree IV in The Silmarillion shows Gundor to the left of Galdor, suggesting that he was the elder brother. In fact, this seems to be merely for convenience of layout, since the text states clearly that Gundor was the younger brother (in chapter 18, Of the Ruin of Beleriand and the Fall of Fingolfin).