Could you build a machine that uses the force of gravity to generate energy?

Sure - one variation of such a machine is called a dam. But you’re falling into a common confusion here, even though you said the right words. Gravity is indeed a force, and that field itself really contains no energy. The field tells us how much energy a mass has by virtue of being at a particular location in the field. For example, it tells us how much energy water behind a dam has. To get that energy and use it, we must let the water fall to a lower elevation. So, the energy is not coming from the gravitational field - it’s coming from the water.

People seem fairly able to grasp this idea - they understand that if they use up all of the elevated water then they can’t get any more energy unless they either wait for rain to fill up the elevated reservoir, or pump some water back uphill (some systems do this - they pump water up during the night when electric demand is low, and then let it fall back down during the day and generate electricity by that process when demand is high).

For some reason, though, people completely overlook the fact that all the same ideas apply to magnetic fields. They somehow think it’s the magnetic field itself which contains energy (and we actually do think of it that way in some cases, though it still comes back to the state of the stuff that’s creating the magnetic field). They think they can use permanent magnets in some clever way to produce endless energy.

This won’t work. The permanent magnets no more provide energy than the hill the dam is associated with does. Just as we have to let material fall down the hill, through the dam and turn generators in order to extract energy, we must let the magnetic field act on something - produce a force on something that’s moving - in order to get energy out. And then once we’ve exhausted our supply of material that the magnetic field can move, that’s it. No more energy, unless we push something back in there so that we can go again. And that “pushing back in” requires us to put in just as much energy as we got out.

So, in spite fo all the cute videos on YouTube purporting to show “free energy devices” built with permanent magnets, there is no such animal.