The present paper shows the results of nanocomposite polymer electrolytes (SPEs) of montmorillonite (MMT; Na+SYN-1) and gellan gum obtained by the solution casting method. The membrane samples were characterized by ATR-FTIR, time-domain nuclear magnetic resonance (TD-NMR), and scanning electron microscopy (SEM). Then, two samples were chosen and applied in small electrochromic devices (ECDs). The ATR-FTIR revealed shifts in bands of acetate and glycosidic bonds at 1032 and 1611 cm−1, respectively, indicating an interaction between the gellan gum and Na+SYN-1. The spin–lattice relaxation time of the proton nuclei (T1) suggests the poor dispersion of MMT in the matrix, especially above 20 wt.%. However, SEM pictures pointed to a more homogeneous surface of the nanocomposite containing 40 wt.% Na+SYN-1 when compared to the sample without clay. NPEs with 10 and 40 wt.% Na+SYN-1 were applied in ECDs, and the voltammograms showed a decrease in anodic and cathodic peaks after 2519 chronocoulometric cycles for the sample with 10 wt.% Na+SYN-1 and after 420 cycles for the sample with 40 wt.% Na+SYN-1. There was also a decrease in the charge density in both ECDs and an oscillating difference in transmittance between the colored/discolored states during the chronocoulometric cycles of the ECD with GG-MMT10. Further studies may reveal more property improvements in gellan gum nanocomposites.