= Solution
At fixed volume and particle number, reversible heating obeys $dS=C_V\,dT/T$. Integrating the <electronic heat capacity> from zero <temperature> gives
$$
S(T)-S(0)=\int_0^T\frac{\gamma T'}{T'}\,dT'=\gamma T.
$$
With no extensive residual ground-state degeneracy, the <third law of thermodynamics> sets the electronic $S(0)=0$. Therefore
$$
\boxed{S_{\mathrm{el}}=\gamma T.}
$$
The <entropy> normalization follows that of $\gamma$: total, per volume or per mole. Lattice contributions are additional and are not part of this electronic result.
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