This study presents investigations on the additive manufacturing of hot work steel with the energy-reduced gas metal arc welding (GMAW) process, which is a cold metal transfer (CMT) process. The paper analyses the influence of arc energy and the thermal field on the resulting mechanical properties and microstructure of the material. The investigations were carried out with hot work tool steel X37CrMoV 5-1, which is used for the manufacturing of plastic moulds, hot extrusion dies, and forging dies. The results show that this steel can be used to generate 3D metal components or structures with high reproducibility, near-net-shaped geometry, absence of cracks, and a deposition rate of up to 3.6 kg/h. The variation of the wire feed speed and the welding speed enables the production of weld beads of width up to 9.4 mm. The mechanical properties of the generated structures can be adapted by the dominant thermal field, which in turn is influenced by the bypass temperature and the electric arc energy. A determining factor to describe the main variables of the welding process is represented by energy per unit length EL. If the bypass temperature is above the martensite start temperature (Ms), there is a homogeneous hardness level along the height of the additively manufactured structure height as long as the energy produced by the welding arc is enough to keep the temperature of all layers above Ms.