Ive been using my vented gas fireplace for a couple weeks not knowing the flue was closed. How dangerous is this. I have myself & 3 dogs that were in my family room with me where the fireplace was burning. I'm worried I've poisoned my poor babies.
If they are not dead, and you are not dead, none of you got poisoned enough to matter. Just don't do that again.
Buy a carbon monoxide detector (or two, or three), and put a note on the fireplace reminding you to open the flue.
Well you and the dogs aren't dead, so that's a good sign.
I wouldn't keep doing it, though. Venting carbon monoxide into your living space isn't great for your health.
Some gas fireplaces have been sold that are designed to be used without venting, on the argument that they aren't producing more CO/CO2 than a gas oven would. I've never been convinced that I believe the argument, and would definitely recommend installing a combined CO/gas alarm as a basic safety precaution; compared to the cost of the gas fireplace, they're a cheap investment.
(I do like a good fire, but not enough that I was interested in the hassles and heat loss of a real fireplace. My current plan is to set up a large flatscreen in a mock fireplace surround and call that good enough. Maybe add an IR heater if I can figure out how to disguise it, to reinforce the illusion.)