In this paper, we describe the design considerations, analysis, and measurement results of two gallium nitride (GaN) digital step attenuator (DSA) designs that cover an ultrawide frequency range between 40 and 220 GHz. We employ a distributed topology and compare designs using either stacked or single-HEMT unit cells in terms of insertion loss, attenuation depth, and linearity characteristics. Both of the designs achieve a nominal attenuation depth of 16 dB in 2-dB steps and a P1dB of 24 dBm at 74 GHz, while the insertion loss up to 220 GHz remains below 4.6 dB (stacked design) and below 6 dB for the single-HEMT designs. These results represent the most broadband GaN-based DSA designs to date.