http://en.wikipedia.org/wiki/Gravity
I think you're being to strict in your question. You're not concerned about the incredible amount we know about gravity, but something you can 'put your finger on."
We know it's generally a choice between curvature of space-time (best choice), a particle or wave force (common, but unsubstantiated as of yet), or Strings (mathemetical). It may even be a combonation of all three. It also may be possible that we can never "know" any more than paint can "know" the whole of the canvas it is painted on.
However, even without 100% absolute certainty into the complete character of gravity, gravity is still real.
Without 100% complete absolute certainty into the complete character of Evolution, Evolution is still real.
Creationism does not have 100% absolute certainty, that's why the substitute knowledge for faith. So if you fault Gravitation, evolution, germ theory, atomic and molecular theory, electron theory, and the host of other theories that explain the ways things occur to you everday; if you fault those for not being certain, you can not hold upon high faith, which is also a lack of certainty. Let alone anything that is born from in uncertainty of faith.