"why is it that when I do work on an object against a gravitational field, that work is stored as gravitational potential energy?".
Simply put, just remember this important rule-the law of conservation of energy-which essentially states that energy cannot be created nor destroyed, it can only be transformed from one form to another. Hence, all objects have stored potential energy when they are not moving, however, from Newton's first law, you would have learnt that all objects will remain stationary unless an unbalanced net external force acts upon that object, and hence that force is essentially what we termed as work done on an object to move if against a gravitational field. It is a bit like trying to swim upstream-you need to put energy to counteract the resistive force of the opposing water current. And essentially, as you move against the gravitational field and you stop at a particular point, the kinetic energy used to move that object to that point is transformed to gravitational potential energy due to the fundamental physics fact that energy is always conserved. Hope this helps a bit.