Ummmm... Not gravity powered
While this light may provide cheap light through mechanical effort, it is a misrepresentation to say that it is "gravity powered." Gravity provides a means to convert potential energy into mechanical energy, but it doesnt' "generate" energy. All of the energy was already in this system.
In this case, the energy is provided by pulling on the rope. Assuming that gravity is actually involved, pulling the rope will lift some weight against gravity, converting mechanical energy (the pull) into potential energy. This is released over time as the weight returns to its starting point, but gravitational potential is acting as a storage for energy, not its source.
