The SASE3 beamline at the European XFEL facility will offer outstanding X-ray beam properties in terms of brilliance and repetition rate in a range of energies that will cover from the machine‘s lower limit, 260 eV, to above 3 KeV. In the first operational phase, it will serve two of the six initially expected scientific instruments: the Small Quantum System (SQS), and the Spectroscopy and Coherent Scattering (SCS) instruments. In each case, the use of a tool that could enable a continuous and smooth modulation of the photon flux is definitely convenient. This purpose can be achieved by means of a gas-based device (so-called “gas absorption cell“) where the attenuation level can be correlated with the actual gas density over a given interaction length and for a given gas species. In addition, the avoidance of solid separation interfaces is also highly desirable, eliminating in such case any optical interference with the beam that could lessen the initial beam quality. The latter condition suggests a windowless system that will have to deal not only with large optical apertures (up to 22 mm diameter) but with pressures up to 35 mbar (8 orders of magnitude higher than the average beamline base pressure). The proposed system has been designed considering not only these factors but others, such as control robustness, gas consumption minimization, gas type flexibility, integration with diagnostics devices, and an evident “user-friendly“ orientation.