Most people would associate "gradient" with its meaning of an inclined surface or the degree of such inclination. Frankly, it doesn't work for me in this sentence. Why not just say "shift" (or "increase" or "reduction", depending on the circumstances).
"Gradient" here implies that you could draw a graph showing clearly the progress and gradual spread of the disease or growth of the organism. In biology the word is used to show successive gradations in metabolism or rate of growth.
So it doesn't mean "shift," which would imply leaving one locus to move to another. It means something closer to line of advancement, i.e. "spread" or Dimcl's "increase."