Gestational diabetes mellitus (GDM) is increasingly diagnosed in women strongly driven by the increase in overweight and obesity in women of childbearing age, yet the literature still addresses this condition more often through the lens of diagnostic glucose thresholds and subsequent nutritional management of glycaemia than through broader concepts of glycaemic dynamics, vascular dysfunction and phenotypic heterogeneity. Both GDM and maternal obesity are associated with altered glycaemia, increased glycaemic variability and endothelial dysfunction within the foetoplacental circulation, with potential consequences for nutrient transfer, foetal growth and later cardiometabolic risk. When these conditions coexist, i.e. gestational diabesity, metabolic and vascular disturbances may be amplified. This review critically examines current evidence on dietary patterns, macronutrients, micronutrients and selected bioactive compounds with hypoglycaemic properties in GDM. Attention is given to CGM-derived indices of glycaemic dynamics and to nutritional mechanisms plausibly linked to endothelial function, including oxidative stress, nitric oxide bioavailability, inflammation and insulin sensitivity. Overall, the evidence suggests promise for higher-quality carbohydrates, dietary fibre, increased protein intake, myo-inositol, vitamin D, selected trace elements, omega-3 fatty acids and probiotics; however, the field is constrained by short interventions, heterogeneous populations, limited use of CGM and a heavy reliance on surrogate metabolic endpoints rather than direct vascular assessment. A more mechanistic and phenotype-aware research strategy is therefore needed before specific nutritional approaches can be considered established tools for improving both glycaemic dynamics and vascular function in GDM and gestational diabesity.