Human milk is a dynamic, bioactive fluid that supports infant growth, development, and immune protection. Gestational diabetes mellitus (GDM), characterised by glucose intolerance with onset during pregnancy, is associated with altered maternal metabolism that may persist postpartum and affect lactation and milk composition. To systematically evaluate and meta-analyse evidence on associations between maternal GDM and human milk composition across lactation, five electronic databases were searched for studies comparing human milk composition between mothers with and without GDM. Data were extracted for milk macronutrients, lipids, oligosaccharides, hormones and metabolic regulatory factors, immune and inflammatory factors, micronutrients and antioxidants, and multiomics across colostrum, transitional, and mature milk. Meta-analyses were conducted where possible. Fifty-two studies with 3233 mothers were included. Macronutrient content, total fat, and fatty acids were found to be similar between GDM and non-GDM milk. However, lactose concentrations were lower in GDM colostrum. In addition, mothers with GDM who were not taking docosahexaenoic acid (DHA) supplements had lower DHA concentrations than mothers without GDM. Appetite- and metabolism-related peptides were lower in colostrum, whereas other regulatory peptides were higher. Mothers with GDM had lower colostrum concentrations of human milk oligosaccharides (HMOs), including 2′-fucosyllactose and 6′-sialyllactose, with partial normalisation in mature milk. Immune, metabolomic, and proteomic findings were heterogeneous but indicated stage-dependent remodelling. GDM is associated with selective, lactation-stage-specific alterations in the bioactive and functional components of human milk rather than the disruption of core nutritional composition. The stage-specific nature of these alterations suggests that human milk composition may adapt to maternal physiological changes during lactation.