I have a GMC-320S, its ok, I had to cut away some of the plastic so it could read a uranium source I bought. The plastic housing would shield the tube from most of the radiation of the source. There are little slits that would let some in, I just widened them with a dremel.

I wouldn't recommend it again, but if you received one as a gift, with a bit of tweaking, you'll be measuring random stuff in your house.