I'm not a big fan of alignment at all and usually reserve it for outsiders. I find it interesting that they may be making Gnolls fiendish.
How I do it:
- Have your mortal races which make up all the various sentient creatures/monstrous and otherwise. You cannot attach alignment to them. To them good and evil is subjective and related to their values and experience.
- Have extra-planar creatures like angels demons and gods, Inevitables etc...(outsiders)
For Outsiders, they are a product of their plane. You cannot have an evil angel or a chaotic modron because 'evil' and 'good', chaos and Law is not a subjective or a behavior but, instead a designation of their status. Essentially, a warm blooded creature cannot be a reptile. If it ever a mammal became cold-blooded, it would no longer be a mammal, it would be a reptile. Hence the same with angels and devils, good/evil.
Now, mortal races are going to emulate certain entities and value specific traits of those entities.
Orcs who were created by Grumsh will tend to emulate and value the traits that embody Grumsh. To most of Orc society, those traits and values are good. The things that Grumsh hates are 'evil'. It Doesn't mean all Orcs are evil or that all orcs emulate or respect those values to the same degree - or at all.
If they are adding Gnolls as fiendish, I'd have those, more broadly, a product of their nature(not nurture): they'd be evil in the perspective of almost all other mortal's sensibilities.