Reliable operation of high-voltage equipment depends on defect-free HV insulation systems. However, defects in the insulation can lead to partial discharge (PD) activity. PD measurement has been shown to be a useful tool for evaluating the condition of high voltage components including gas-insulated switchgear (GIS). This work focuses on characterizing the motion and PD signal behavior of free-moving particles in an experimental GIS arrangement under high dc field stress. For this purpose, the particle geometry, material composition, and amplitude of the applied negative dc voltage are varied. PD signals are measured using a broadband ultra-high-frequency (UHF) sensor and recorded in the time domain. The UHF signal is used to trigger a synchronized high-speed camera in order to observe the correlation between particle motion and UHF PD pulse activity. Certain unique combinations of UHF pulse repetition rates and signal amplitudes in the time domain are observed that depended on and were correlated with particles’ motion. These are evaluated by means of pulse sequence analysis, whose significant clusters enable estimation of particle motion. In particular, particle motion with small hopping amplitudes on the negative electrode—known as “firefly”—is differentiated with high accuracy.