Years ago it was common to see in many houses those open, grated gas
heaters. I once lived in an apartment which had one of these heaters in
every room. As I recall, they weren't even vented. And yet I don't recall
any warnings against using these heaters, nor do I recall any fatalities.

I've been thinking of installing one of these -- you can still find them in
various places -- as a substitute for a wood fire.

Is there a danger?

Also, is it possible to use a flexible gas supply line so that these heaters
can be moved around. I'd like to put it in the fireplace when it's not in
use, and pull it out onto the hearth when it is in use.