I’m under the impression the greek word for magician or sorcerer is “magus”. I have no clue what “goistais” is ...
That's Latin I think.
My Liddell and Scott Greek Lexicon has gohs, gohtos, (goes, goetos) "one who howls out enchantments, a sorceror, enchanter". I dunno where that "s" comes from though, unless it is some dialect besides Attic.
Yes, magus (μάγος) is correct.
There doesn't seem to be any online reference to "goistais" aside from stories related to this find.