This article focuses on the new generation of Very Long Baseline Interferometry (VLBI), the VLBI Global Observing System (VGOS), and measurements carried out during CONT17 campaign. It uses broadband technology that increases both the number and precision of observations. These characteristics make VGOS a suitable tool for studying the atmosphere. This study focuses on the effects of the ionosphere on VGOS signals using a model that incorporates and extends ideas originally published in Hobiger et al. (2006). Our investigation revealed that the differential Total Electron Content (dTEC) data product calculated with the VGOS post-processing software revealed a sign error in the inferred dTEC values. Fortunately, this error does not change the final values of the phase and group delay. Therefore, this study was a way to identify this problem within the VGOS data. After diagnosing and solving this problem, the Hobiger et al. (2006) model was modified such that instead of considering a single unknown for the latitude gradient of the ionosphere, a time series of latitude gradients were taken into account. For comparison purposes, time series of Vertical Total Electron Content (VTEC) at each station during CONT17 campaign were constructed from Global Ionosphere Map (GIM) data. In this way, it is shown that the final accuracy of estimating VTEC at each station using VGOS data was between 1.0 and 5.8 TEC Units (TECU).