Lacunar spinels $\mathrm{A}{\mathrm{M}}_4{\mathrm{X}}_8$ ($\mathrm{A}=\mathrm{Ga}$, Ge, Al; $\mathrm{M}=\mathrm{V}$, Mo, Nb, Ta; $\mathrm{X}=\mathrm{S}$, Se) have garnered significant attention due to their remarkable properties, including multiferroicity, Mott-Hubbard gaps, and skyrmion phases. In this study, we present temperature-dependent Raman spectroscopy measurements of $\mathrm{GaM}{\mathrm{o}}_4{\mathrm{S}}_8$, spanning the Jahn-Teller (JT) distortion and spin ordering transitions. Using group theory analysis and first-principles calculations, we identified and assigned ten Raman-active modes, among which the two lowest-energy modes, $A_1^1\left(T_2^1\right)$ and $A_1^2\left(T_2^2\right)$, were confirmed as JT active modes. The $A_1^2\left(T_2^2\right)$ mode exhibits a redshift as the temperature approaches 50 K from both sides, indicative of a soft mode and suggesting a displacive nature for the ferroelectric transition. Additionally, anomalies observed in the $A_1$ and $E$ modes near 50 K and 25 K correlate with the JT transition and subsequent magnetic ordering, pointing to significant electron-phonon and spin-phonon coupling in $\mathrm{GaM}{\mathrm{o}}_4{\mathrm{S}}_8$. These findings deepen our understanding of the mechanisms of the phase transition and the orbital-spin-lattice interactions in $\mathrm{GaM}{\mathrm{o}}_4{\mathrm{S}}_8$, while highlighting the crucial role of phonon dynamics in the $\mathrm{A}{\mathrm{M}}_4{\mathrm{X}}_8$ family.

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• Electron-phonon coupling
• Ferroelectric phase transition
• Jahn-Teller effect
• Spin-phonon coupling
• First-principles calculations
• Raman spectroscopy