What is the mass loss of the nucleus, in u, upon emission of this gamma ray? -g?

Suppose that the energy of the emitted gamma ray photon is given the symbol "e" in units of Kev
Now, we use Einstein's equation (E = mc² ) where c is the speed of light (3 x 10^8 m/sec)

substituting in this equation, we find that:
e = mass x c² 
mass = e / c² 
Now to get the corresponding value in atomic mass unit:
1 eV/c² = 1/(931.46 * 10^6) amu 
Therefore,
The mass in amu = e x  1/(931.46 * 10^6) atomic mass unit