It's a known bug... I had the same issue. Reported it and TS are already aware. They did send me another gnome but I experienced the same issue : ( : ( You'll just have to keep it inside your house for the time being (or sell it...which is what I ended up doing!)
I bought a gnome for my old house and I couldn't put it down by my front door.
Then I bought a bigger house. I found that it had a gnome by the door already. That gnome treats me like a stranger! I can't pick it up and I can't teach it anything new to say. I am pretty unhappy about this because it is saying something unfriendly. :(