Three things people get wrong in the first two minutes, and all three can be fatal: what to touch when you smell gas, how to recognise carbon monoxide poisoning before someone collapses, and what an alarm actually protects you from. Pick the situation you are in.
CO is colourless, odourless and tasteless, and it is produced by any fuel burning without enough oxygen — gas, oil, coal, wood, paraffin, petrol, charcoal. The "gas smell" people recognise is not the gas; it is a chemical deliberately added so you can detect a leak. Carbon monoxide from a badly burning appliance has nothing added to it at all, which is why a working alarm is the only reliable way to know.
CO binds to haemoglobin roughly 200 times more strongly than oxygen does, so it takes the blood's carrying capacity away and the body does not notice until quite late. Early symptoms — headache, tiredness, nausea, dizziness — look exactly like a virus, a hangover, food poisoning or stress. The pattern that gives it away is that everyone in the house feels it, including pets, and it improves when you go out.
UK numbers, worth checking locally: 999 for anyone who is unwell or unconscious, 0800 111 999 for the National Gas Emergency Service. Landlords in England must have a working carbon monoxide alarm in any room with a fixed combustion appliance; tenants are still entitled to ask, and to report a fault.