The name 'Gillie Dubh' - sometimes given as Ghillie Du - literally means dark [dubh] lad or servant [gillie] but the 'dark' here refers to hair color not temperament or nature. Briggs points out that this fairy, who is considered ubiquitously male, was called Gillie Dubh due to his dark hair and not his clothing (Briggs, 1976). This is a reflection of the common practice in both Irish and Gaidhlig of referring to a person's hair color by calling the person themself by that color, hence 'dark lad' a dark-haired lad, 'red woman' a red haired woman. The Gillie Dubh is said to dress entirely from forest flora, specifically leaves and moss (Briggs, 1976).
During the 18th century he was commonly known in one area of Scotland, and as one author put it: "he was seen by very many people and on many occasions over a period of more than forty years in the latter half of the 18th century." (MacKenzie, 1921, p234). He is most strongly associated with the area of Gairloch in what used to be Ross and Cromarty [now Wester Ross] and further north particularly with the area around Loch an Draing (MacKillop, 1998; MacKenzie, 1921). His preferred home is birch groves, of which he is the special guardian, and one might surmise that it is the leaves of this tree that he prefers to dress.
In at least one story he sheltered a lost child in the woods at night and then to have brought her home the next day; this girl is also the only living person the Gillie Dubh is ever known to have spoken to (Briggs, 1976). Some modern folklore suggests that the Gillie Dubh aids lost travellers, likely rooted in this older anecdote. His overriding characteristic however is his reclusive nature and reluctance to engage with humans.
By the measure of the Scottish fairy court system the Gillie Dubh would be considered a Seelie court fairy. Despite this there is one story of a group of Scottish lords in the 18th or 19th century who set out to hunt him down (Briggs, 1976; MacKenzie, 1921). Despite thoroughly hunting the woods and loch area they found nothing and left empty handed (MacKenzie, 1921). No reason is given for this decision, as it's clear the fairy wasn't harming or even harassing anyone, and even more oddly the people who did this chose to stay with the woman who had been rescued from the woods by the Gillie Dubh, now a married adult. It seems that after this time the fairy stopped appearing so often or easily to people as he had previously.
Briggs, K., (1976) A Dictionary of Fairies
MacKillop, J., (1998) Dictionary of Celtic Mythology
MacKenzie, O., (1921) A Hundred Years in the Highlands