The best replacement for gamma is brightness.

The best alternatives to gamma are rated as following:
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★★  beta radiation
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★★  contrast
★★  x ray
★  vega
★  akrho
★  alpha motor neuron
★  theta
★  lognormal
★  xray
★  neutron radiation
★  alpha kappa rho
★  p90x3
★  ebeam
★  apo
★  klippermate
  alpha motor neurons
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  convexity
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  omega sonic
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  akro

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Speaking about gamma, one probably shouldn't forget about 002.

Other search terms related to gamma and their ratings are:
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gamma
brightness
beta radiation