No, you're probably hearing it just fine. Thing is, in the untranslated novels, the spell's name is written as 奈落 (naraku--"hell", though it could reasonably be translated as "the abyss" if you needed), but the furigana tell you to read it as ギンヌンガガプ (Ginnungagap, the abyss of Norse mythology). In these situations, the anime would usually go with the suggested reading, but I guess they decided it was too much of a mouthful or something.

(Bear in mind, it's been a while since I last watched the anime.)