Introduction and objective: Gestational diabetes mellitus (GDM), in case of inadequate glycaemic control, is associated with an increased risk of maternal and neonatal complications. The aim of this study was to evaluate data concerning outcomes of neonates born to mothers with GDM, treated with only diet (GDM G1) compared to diet and insulin therapy (GDM G2). Special consideration was given to data concerning outcomes of neonates born after the COVID-19 pandemic outbreak in 2020. Materials and methods: Medical records were analysed. The data comprised records of newborns born at term or near term in the years 2017–2021. The statistical analysis was performed, with the significance level set at α = 0.05. Results: A total of 646 medical records were analysed. The study groups differed significantly (p < 0.05) in the length of hospitalisation, incidence of cardiological abnormalities, and method of feeding during hospitalisation. There were no differences (p > 0.05) between the groups in the results concerning neonatal anthropometric measurements, general condition after birth, or other perinatal outcomes. In 2020, significantly more (p < 0.05) neonates required support in their adaptation period. These neonates were more often (p < 0.05) diagnosed with birth defects and other complications. Conclusions: The level of maternal glycaemic control, appropriate medical care, and thus effectiveness of maternal treatment are important factors influencing neonatal outcomes and breastfeeding rates.