I have vented my gas dryer into my Minnesota house, where I raised my three children, for 40 years with no adverse affects. Never had a problem with humidity as many have suggested. Never had a problem with micro-lint as many have suggested. The dryer's screen removes 98% including most micro once the initial lint film is built up. Pantyhose over the discharge pipe removes the rest. Never a dust/lint buildup on surfaces in the house much less in the laundry room.
I suspect most on this site that have commented on this question have never done it. Having several CO detectors will protect from a faulty appliance. In addition I have never seen a comment on the expense of having to reheat outside air, often at zero degrees or lower, that gets sucked in through windows and doors as makeup air. So - my question is, based on my experience there being no issue with humidity or lint in cold winter months, how is this any different than a gas range/oven that vents directly into the kitchen with no questions asked? And shouldn't not having to reheat makeup air with your furnace be a factor?
Considering all the negative affects from climate change, this issue should be revisited.