A bag you fill and hang, with a filter and a hose underneath. It processes several litres unattended, which is a different proposition from a pump or squeeze filter that you operate by hand for each litre.
That difference is the whole reason to own one. If you are moving every day and filling a bottle at a time, a squeeze filter is lighter and simpler. If you are staying put and cooking for several people, the gravity bag runs while you get on with dinner.
It is also the more sensible choice for the household case. During a mains interruption or a boil-water notice, a gravity filter turns a bathtub or a water butt into something usable at a rate that keeps up with a kitchen, which a squeeze filter does not.
What it does and does not remove is the part to check, and it is the same answer as any filter: pore size determines organisms, and chemical contamination is a separate question that filtration may not address. The article is water sourcing and treatment, and it is worth reading before choosing, not after.
Look at the flow rate and the stated cartridge life, and check the hanging arrangement works where you would actually use it. A bag needing a branch is useless in a car park.