Critical behavior of the infrared reflectivity of ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$ ceramics is observed near the superconducting ${T}_{\mathrm{SC}}=45 \mathrm{K}$ and magnetic ${T}_{M}=133 \mathrm{K}$ transition temperatures. The optical conductivity reveals the typical features of the c-axis optical conductivity of strongly underdoped multilayer superconducting cuprates. The transformation of the Cu-O bending mode at $288 {\mathrm{cm}}^{\ensuremath{-}1}$ to a broad absorption peak at the temperatures between ${T}^{*}=90 \mathrm{K}$ and ${T}_{\mathrm{SC}}$ is clearly observed, and is accompanied by the suppression of spectral weight at low frequencies. The correlated shifts to lower frequencies of the Ru-related phonon mode at $190 {\mathrm{cm}}^{\ensuremath{-}1}$ and the mid-IR band at $4800 {\mathrm{cm}}^{\ensuremath{-}1}$ on decreasing temperature below ${T}_{M}$ are observed. It provides experimental evidence in favor of strong electron-phonon coupling of the charge carriers in the Ru-O layers which critically depends on the Ru core spin alignment. The underdoped character of the superconductor is explained by strong hole depletion of the ${\mathrm{CuO}}_{2}$ planes caused by the charge carrier self-trapping at the Ru moments.