The most abundant isotope is Gz-83 because the average atomic mass of Gz is closer to 83.
The average atomic mass is defined as the weigthed mean of the isotopes.
The mass of the isotopes is 80, 81 and 83 uma, respectively.
As the average atomic mass (82.74uma) is closer to the atomic mass of Gz-83 than the mass of the other isotopes, you can interpretate that the most abundant isotope is Gz-83.
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