Recently, there has been much interest in the capability of low cost glow discharge detectors (GDDs) for detection of terahertz radiation. To utilize them in applications such as terahertz imaging these studies have typically focused on the response of the GDD at specific frequencies. To better understand the spectral behavior of glow discharges, we examine the interaction mechanism of GDDs with terahertz radiation using terahertz time domain spectroscopy in a broader range of frequencies between 0.05 and 0.5 THz. Our results show that in addition to terahertz-induced oscillations in the plasma charge density, the structure of the GDD itself plays an important role in the detection mechanism. We observe that by increasing the bias voltage on the gap, not only can we greatly reduce the transmission at a specific frequency, but we can also tune it as well, suggesting that these devices can be used as hybrid plasma filters.