You are actually looking for a force sensor, of which the mentioned strain gauge is one type.
But if you are trying to measure punches, then you need to have a sensor with enough bandwidth to track the applied force on a millisecond basis (at least). For example, the entire punching bag can tell you the total transferred energy, i.e. integral of force over time, simply by applying pendulum equations. But that will not tell you much of anything about the momentary peak force applied.
Next problem is that punching bags are soft, so the force is quickly dispersed from the impact area (fist cross-section) to a much wider area. You probably will have to mount the sensing element on a small rigid plate on the surface of the bag to get an accurate force (pressure times area) reading.