Advanced energy transmission presents ambitious challenges to the required space of installation and transmission capacity. In this context, gas-insulated systems for HVDC operation enable space-saving, low-loss and reliable realizations. Development and dimensioning require comprehensive understanding of the insulation system, considering relevant influencing factors in the gas, in the solids and at the gas-solid interfaces. Detailed knowledge allows developing a computational model for different operating conditions, depending on load and ambient temperature. This contribution deals with material properties, major influencing factors, test and measurement set-ups and presents results from experiments supporting a reliable design of gas-insulated DC systems. All investigations show that a comprehensive knowledge of the gas-solid insulating system is the key factor to design robust and compact components for gas-insulated HVDC systems.