A set-up for optical gas composition measurement based on absorption spectroscopy is composed of a white light source, a gas cell and a spectrometer. The Fabry-Perot optical filter is suitable for miniaturization of this system, as it is composed of only two reflectors with a transparent layer in-between. Varying the width of this optical resonator to cover the wavelength range in which the components to be analyzed have specific features, gives the absorption spectrum. The gas cell remains a separate unit in the conventional microsystem and a MEMS is the implementation of the microspectrometer. The ultimate miniaturization requires the monolithic integration of all these components. In the lossy Fabry-Perot filter that is presented in this paper, the gas cell and the microspectrometer are functionally integrated, using a gas-filled resonator in which the optical path is determined by multiple reflections in the resonator cavity itself.