Gliese is what happens when the Twin Souls fandom somehow finds out about an association between Alcor and a small, red-haired child who wields blue fire and starts speculating real-life “headcanons” about said child’s identity and relation to the demon and eventually, generally comes to the conclusion that it must be the offspring of Alcor and Mizar, and that said child may be a shapechanger to boot since there are slightly differing descriptions by witnesses of said child, though all involved red hair.

Basically some people who didn’t know better occasionally saw Willow with her uncle, and some others might’ve seen Acacia or Hank a time or two instead, and everything got mashed together in a kind of combination urban legend and fandom’s headcanon, and Gliese is just the fan-made name that rose to greatest prominence and popularity, since nobody knew otherwise.