Gestational diabetes (GDM) is quite common during pregnancy, and its prevalence is rising because of the increased overweight and obesity rates. In patients with GDM, proper glycemic control, adherence to a suitable diet and antidiabetic treatments can reduce the likelihood of maternal-neonatal complications. For this reason, this study aims to assess the therapy adherence of pregnant women with GDM. Treatment adherence was assessed by both glucometer and diabetologist’s analysis reported in the electronic medical record. Cohen’s Kappa was used to assess the agreement between the two classifications. Moreover, a multivariate logistic regression analysis was performed to identify potential risk factors for non-adherence to treatment. Overall, 287 patients were enrolled, and 271 were available for follow-up. Low concordance between the glucometer and the diabetologist’s analysis was found, mainly due to the complexity of patients with GDM. Indeed, 46% of patients were classified as not adherent due to glucometer results and 42% based on medical assessment. This study highlights the importance of monitoring patients with gestational diabetes to assess and increase adherence to therapy properly.