Compared to other forms of electromagnetic radiation, Terahertz (THz) radiation is considered safe for imaging and detection purposes due to their non-invasive and non-destructive nature. Novel techniques and methods are constantly being evaluated for efficient detection and measurements of THz waves, yet their commercial success is limited due to cost and overall complexity of these systems. Commercially available Glow Discharge Detectors (GDDs) are proven to detect microwave and higher frequency radiation. Although, the plasma generated inside the GDDs enables the detection of the EM radiation, the interaction mechanism is not fully understood. In this study, we investigate various types of GDDs that are commercially available, and characterize their overall behaviour in response to both CW and modulated EM radiation. In addition, the response from various GDDs to the mm-wave/THz radiation are surveyed for the range of 260-380 GHz. Measurements show that response signal at certain frequencies within the range are attenuated suggesting a resonance frequency possibly based on the electrode structure.