Ability rip causes its target, should they lack supernatural abilities, to permanently 2 class levels, or 2 HD if you've no class levels (what happens if you have exactly 1 class level is addressed here). Assuming a character with 2 HD and no supernatural abilities, like a typical gnoll, is targetted by Ability Rip the result is a 0 HD gnoll. Since the typical gnoll also has no class levels and isn't a lycanthrope, this results in a creature with 0 total HD from all sources. How does that work? If the creature is also a PC and playing a LA:+0 race like, e.g., an Anthropomorphic Dromedary Camel, how much XP do they need to reach ECL 1? How much XP does an APL 0 get from a level-appropriate encounter? Basically, how does 0 HD 0 class level PC life work differently than normal PC life?
The creature dies.
Ability rip, shockingly, is poorly written. Serpent Kingdoms wholesale is poorly written. Unlike every other source of level loss—mostly negative levels, but also Savage Species rituals—ability rip fails to explicitly indicate that the creature dies. Very telling that Serpent Kingdoms is even less careful than Savage Species.
But the creature dying is the only result that is consistent and sensible. The game does not support 0-HD creatures; having HD is one of the things that makes a creature a creature. Even for some RAW theoretical optimization exercise, the best you can get out of this situation is “undefined,” not any particular result you might be hoping for.