We present a gastric gas sensor based on conjoined dual optical fibers functionalized with sensitive optical dyes for sensing gases in both fluidic and gaseous environments. The sensor aims to sense various concentrations of carbon dioxide (CO ₂ ) and ammonia (NH ₃ ), which are two significant biomarkers of H. pylori infection in the stomach. It is known that CO ₂ and NH ₃ are released during the hydrolysis of urea by H. pylori, a bacterium that may cause stomach cancer with relatively high probability. CO ₂ and NH ₃ sensitive optical dyes, cresol red ion pair and zinc tetraphenylporphyrin, are embedded in silica beads and then functionalized onto the thin PDMS-coated fiber tip. Each type of dye provides a unique spectral emission response when excited with light ranging from 450 to 700 nm. Two SMA connector legs of the as-functionalized sensor are connected to an external light source for illumination and a ultraviolet-visible-near infrared (UV-Vis-NIR) spectrometer for signal collection/readout. To perform the measurements, one fiber illuminates while the other fiber collects the back-scattered light and feeds it to the UV-Vis-NIR spectrometer to measure the change in light spectrum as a function of CO ₂ or NH ₃ concentration. This method is easy and flexible and achieves ppm level sensitivity to targeted gas analytes. The proposed sensor can be integrated into a customized tethered capsule for adjunctive diagnosis of H. pylori infection to improve the accuracy of visual endoscopic inspection.