Power stress mark is crucial for estimating Thermal Design Power (TDP) of GPGPUs to ensure efficient power control. This paper proposes Guser, the first systematic methodology to generate GPGPU power stressmarks. It features three ideas: Instruction Power Analysis (IPA) to analyze the power behavior of PTX instructions; Pipeline-based Instruction Grouping (PIG) to classify all PTX instructions into a number of groups; and Quantifying the Importance of power impact factors (QIF) to select a small number but important adjustable knobs. We adopt Optuna, an advanced BO algorithm, to generate stressmarks that maximize power consumption. We evaluate Guser on two real GPGPUs Tesla T4 (Turing) and Tesla A10 (Ampere). The experimental results show that the Guser-generated stressmark for T4 (T4-stresser) consumes 109.3 watts and the one for A10 (A10-stresser) consumes 238.7 watts, which are 48.7% and 73% higher than those consumed by the stressmarks generated by the state-of-the-art approach for CPUs, respectively. Moreover, T4-stresser and A10-stresser consume significantly more power than that consumed by any benchmarks in three benchmark suites: Cactus, Rodinia, and Parboil.