= Solution
Apply the residue theorem to $f(z)/(z-w)^{n+1}$; its residue at $w$ is $f^{(n)}(w)/n!$, which proves Cauchy's <derivative> formula.
For a compact subdisc choose a slightly larger contour still inside $D(a,R)$. Cauchy's <derivative> formula bounds the supremum of $g_k^{(n)}-g^{(n)}$ on the smaller disc by a fixed constant times the supremum of $g_k-g$ on the contour. Local <uniform convergence> therefore passes to every <derivative>.
On $|z-a|=\varepsilon$, $g$ has a positive minimum <modulus>. For large $k$, $|g_k-g|<|g|$ there. Rouché's theorem then says $g_k$ and $g$ have the same number of zeros inside, namely $m$, counting multiplicity.
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