Its a while since i did the planetary geology stuff (I've moved onto more local hydrocarbon exploration since), but in answer to the above;
The gravel is dense enough and rounded enough to prove transportation by a viscous medium.
Mars likely stopped its tectonism a long time ago, which meant it stopped producing gasses able to replenish the lossese to space. On earth, the serendipity of life - first anaerobic, then aerobic - created an atmosphere that sustains an atmosphere and, unlike Venus, is able to achieve equilibrium with solar energy.
Life may be present in this stream by NASA, but its more likely to be found in the places where he most basic life can develop, such as the clays and mineral rich sedimentary deposits thought to exist elsewhere in Gale crater. Proof of life doesn't need to be a fossil encased in gravel, just a complex amino acid or RNA which is unlikley to be formed through conventional physical chemical processes.
The really intersting science about Mars (for me at least) isn't the search for life - surely life can develop elsewhere, but it needs to be intelligent or terraforming (re: Earth) to be of interest - but the internal structure and understanding why tectonism stopped. This applies even more so for Venus.
Beer icon because you shouldn't write on t'internet after the pub....