That's a pretty cool and flavorful way of doing it. Gnomes as a fungal infection on orcs? Orcs as parthenogenic? Awesome.
Now, since orcs are listed as female, does that mean they can reproduce with humans (or others)? Or do they just have a female appearance?
ETA: As an addendum... while it is not MY problem, I imagine some people will have objections to calling them "orcs" and "gnomes" because you are changing them so radically. I'd ignore those people; the terms mean what you want them to mean.