Also a good point. Though there's a chicken-and-egg problem here... is Gruumsh evil because orcs are evil? Or are orcs evil because they were created by Gruumsh? The latter choice seems better supported by the usual fluff. (Though if you go the Tolkien route, perhaps Gruumsh 'created' orcs by corrupting them from some other, relatively nice, raw material.)
This depends on how orcs are built in your campaign. In some, Gruumsh has created them, in some others, he did not.
For example, in most fictions I can think of, Drows *weren't* created by Lloth. They were created by Corellon, and then split from the "good elves" for some reason (which vary from world to world). They worship Lloth because they are evil, not because they were created like that.
However, Evil Gods creating Evil Races also work as a rationalizing about why such evil races exist. I mean: in real world, no culture or race is intrinsically evil to it's roots. Sure, they might have had "not-so-bright" parts of history here and there (such as Germany's nazism, Spainish Inquisition, or the Slavery in the south of US), but the people is not "evil" in the sense that gnolls are. They are just misguided, confused, or have a moral compass that looks bad now, but didn't in their era (Inquisitors would had described themselves as "lawful good" for "fighting witchcraft").
That said, even with the difference between real world and D&D pantheons... "turn undead" is not a classic trope in every culture of Earth. In some of them, particularly those of judeo-christian-islamic heritage, yes, it does, because God is only one, and he is good, and fights The Devil which is bad, so is all witchcraft and wizardry. In other traditions, it doesn't work that way. Chinese Vampires are hold with mirrors, or using what would be described as "wizardry" by western observers (I Ching rituals and what not). In Voodoo religion, Zombies are not only not held at bay by houngans, but actually *created* by them. In Rome, one wouldn't pray to Poseidon for protection against undeads, for the same reasons one wouldn't pray to Hades for protection against sea monsters.
And so on. Turn Undead isn't a mechanic that all the religions should share, imho, because not all religions do so in real life myth. It's something the game carries on, since the times where "clerics" where modeled after christian templars. Just like they had proficiency with heavy armors and shields and maces. It made sense, somewhat, in that era, but when 2e introduced the specialty priests, those where built under specific rules, depending on the god. They had something diferent, instead of turn undead (like Incite Berserker Rage, Charm, or Inspire Fear were the examples provided for War, Beauty and Death gods), and the weapons and armor proficiencies depended on the god too. When 3e came, this broke apart, and all specialty priests where given Turn Undead and full armor proficiencies, so the clergymen of Nature and Wisdom walked the world turning Undeads and wearing full plate.
. However, other undeads aren't scared or turned away in traditional stories -zombies , for example-. Turn undead isn't a common trope out of european judeo-christian tradition, either. It's not common to see "Van Helsing-like" stuff in mythology about undeads in Japan, or China, or India, or the Aztechs.
