A carbon dioxide (CO₂) sensor based on gas-tuned surface plasmon polariton (SPP) propagation length is proposed using hybrid multilayer silver-dielectric structures integrated with polyhexamethylene biguanide (PHMB). The sensing mechanism exploits the CO₂-induced change in the refractive index of PHMB to modulate the effective complex propagation constant and, consequently, the propagation length of SPPs. When the CO₂ concentration increases from 0 ppm to 524 ppm, the refractive index of PHMB decreases from 1.55 to 1.48, corresponding to a relative change of approximately 4.5%. To investigate the enhancement of this small refractive-index variation, single-, double-, and triple-layer silver structures are systematically studied using the transfer matrix method and finite-element simulations implemented in COMSOL. The effects of silver-layer thickness, dielectric spacer thickness, operating wavelength, and the number of silver layers on the SPP propagation length variation ΔL and relative change S are investigated over the wavelength range of 500-1500 nm. The calculated results show that the propagation length increases as the refractive index of the PHMB layer decreases, providing a direct optical response to CO₂ concentration. Importantly, the relative variation of the SPP propagation length is substantially larger than the 4.5% refractive-index change of PHMB, and under optimized structural parameters, the propagation-length variation can reach approximately 27%, corresponding to an enhancement of about six times the original refractive-index variation. The single-layer silver structure exhibits the largest propagation-length variation, while thinner silver films generally provide stronger responses. Meanwhile, longer wavelengths lead to a larger absolute propagation-length variation but a lower relative change, revealing a trade-off between signal magnitude and relative sensitivity. For double- and triple-layer structures, the influence of silver thickness is reduced owing to interfacial near-field coupling, whereas increasing the dielectric spacer thickness can induce mode evolution and decoupling of the coupled SPP modes. Based on the obtained parameter dependence, a single-layer Ag/PHMB structure with a 100 nm silver film and an operating wavelength of 850 nm is further considered as a practical sensing configuration. The calculated propagation length increases from 14,829 nm to 17,341 nm as the CO₂ concentration increases from 0 ppm to 524 ppm, corresponding to a propagation-length sensitivity of approximately 4.79 nm/ppm. These results demonstrate that the proposed gas-tuned propagation-length sensing strategy can effectively amplify a small CO₂-induced refractive-index variation and provides a promising approach for sensitive and real-time CO₂ detection.