The development of high-voltage direct current gas-insulated switchgear assemblies (DC GIS) of rated voltages up to ± 550 kV has been completed. DC GIS provide a compact technical solution with a high functional density, optimized for projects with limited space as in offshore HVDC converter platforms, onshore HVDC converter stations and transition stations between different transmission media. Up to now, no standards for testing of gas-insulated DC systems are available, although pre-standardization work is in progress within CIGRE. Some tests can be performed as required in AC GIS standards. Special aspects of DC voltage stress, like the electric field distribution of insulators influenced by the accumulation of electrical charge carriers and the operation-related inhomogeneous temperature distribution, must be considered by additional electric and thermoelectric tests. For DC GIS, the experience of long-term performance is limited today. Although ageing is expected to be of lower importance, tests are recommended. This contribution summarizes the physical and technical background to design and develop compact DC switchgear assemblies using gas-insulated technology. It explains the developed modules of the substation and gives an overview of the performed tests. Furthermore, it provides an insight in the on-going standardization activities and describes applications in converter and transition stations, showing its space-saving characteristics.