This study advances distributed sensing with optical fibres by proposing gas-mediated platforms that overcome two long-standing limitations of silica-based methods: the unavoidable cross-sensitivity between temperature and strain, and the loss of reliable temperature sensitivity at low temperatures. Three studies based on distributed sensing in gases are developed and experimentally validated. First, a threshold-temperature alarm is realised by filling a side air-hole optical fibre with carbon dioxide and interrogating it with conventional optical time-domain reflectometry. When local temperature falls below a pressure-set point, carbon dioxide condenses inside the holes and liquifies, sharply increasing local optical loss while preserving core guidance. The resulting change in the backscattered trace identifies the position of cold or hot spots with simple hardware, and the contrast can be tuned by selecting the optical wavelength. This provides a robust, multi-point, distributed event detector rather than a full thermometer. Then, to eliminate the temperature cross-sensitivity in the strain sensing based on phase-sensitive Rayleigh scattering, a solid-core photonic-crystal holey fibre is filled with a gas at controlled pressure so that the negative thermal response of the gas compensates the positive thermo-optic response of silica. Operating near this athermal point yields a strain-only readout while retaining strong coherent Rayleigh backscattering signal from the solid core. The analysis quantifies how the compensation depends on gas species, pressure, wavelength, and modal overlap, and demonstrates dominant strain sensitivity with greatly suppressed thermal drift. Finally, this study establishes an absolute, calibration-free thermometry based on stimulated Brillouin scattering in gas-filled hollow-core fibres. The Brillouin frequency shift, linewidth, and gain can be theoretically predicted based on the thermodynamic gas state equations at given temperature and pressure. Experiments with neon, argon, and nitrogen across 67-350 Kelvin confirm these predictions with high agreement (without calibration), showing a monotonic sensitivity increase with decreasing temperature, and demonstrate distributed temperature measurements with sub-Kelvin accuracy using the Brillouin echo scheme. Overall, this study presents the following: i) a simple distributed temperature alarm for threshold crossings; ii) a phase-sensitive Rayleigh system with intrinsic strain-only response; and iii) an absolute Brillouin thermometer that remains accurate and highly sensitive in the cryogenic regime. In summary, gas-mediated fibre sensors extend the scope of distributed sensing technology, transcending the inherent limitations of solid silica and offering tuneable physical properties through gas selection, pressure, and waveguide design.