Volcanic eruptions provide broad spectral forcing to the atmosphere and many previous studies have examined the IT disturbances caused by volcanic eruptions through both observations and modeling. Understanding the primary mechanisms that are relevant to explain the variety in waveform characteristics is still an important open question for the community. In this study, Global Navigation Satellite System (GNSS) Total Electron Content (TEC) data are analyzed and compared to simulations performed by the Global Ionosphere-Thermosphere Model with Local Mesh Refinement (GITM-R) for the first phase of the 2015 Calbuco eruption that occurred on 22 April. A simplified source representation and spectral acoustic-gravity wave (AGW) propagation model are used to specify the perturbation at the lower boundary of GITM-R at 100 km altitude. This modeling specification shows a good agreement with GNSS observations for some waveform characteristics such as travel/onset times and relative magnitudes. Most notably, GITM-R is able to reproduce the significance of AGWs as a function of radial distance from the vent, showing acoustic dominant forcing in the near field (<500 km) and gravity dominant forcing in the far-field (>500 km). The estimated apparent phase speeds from GITM-R simulations are consistent with observations with ~10% difference from observation for both acoustic wave packets and a trailing gravity mode. Relevance of the simplifications made in the lower atmosphere are then discussed and test changes to the assumed propagation structure, from direct propagation to ground-coupled propagation, show some improvement to the data-model comparison, especially the second acoustic wave-packet