For anyone who has an understanding of ancient Greek language ( Bryan Don Eikadistes ...would the Garden actually have been referred to as "Ho Kepos" or just "Kepos"?
The way I look at it (again, total amateur here), the article ὁ helps indicate that "the Garden" is a proper noun, the name of some unique place, as opposed to a "garden", which could be one of many. I imagine that Athenian Epicureans could have casually disregarded the article, but it's sort of like ... "The Beatles". You'll always put "The" in front of the brand unless you are referring to each "Beatle" individually, or the group of those four people as "Beatles", but the band, proper, is "The Beatles", and it indicates more than just a group of four people; it indicates their entire brand.