You can't be gnomes in PoR?

The FAQ says:
Q: What are the races and classes?
A: We are supporting the basic Classes and Races per the new 3rd Edition Rules: Fighter, Barbarian, Paladin, Ranger, Cleric, Sorcerer, Monk, and Rogue. Races we’ll support are Half-Orc, Human, Halfling, Dwarf, Elf, and Half-Elf. This is not to say that these are all of the races in the game; we’ll have Drow NPCs and other races as well.

Is there a mistake or did they segregate gnomes?

They did...
When they started to make the game, gnomes were not a race in 3rd edition D&D. Then it was too late to add them in. So they say.

Not that anyone will miss them much. Or are you playing one BH? ;)

shttt.. yah kinda.. I like playing them cuz they uhm are challenging because of the low hitpoints (I'd play halflings but I'm not a fan). Normally, I play an elf, but I have this kick arse idea for a gnome illusionis (I prefer sorcerer, but I can't remember if you can choose that class with a gnome.. it just seems like you can be anything with 3rd Ed you can do anything. It is great fun.)