Gestational diabetes mellitus (GDM) is a heterogeneous metabolic disorder in which maternal dysmetabolism interacts with placental adaptation and fetal development. Plant-derived bioactive compounds have attracted interest because they modulate metabolic, inflammatory, redox, mitochondrial, nutrient-sensing, and vascular pathways implicated in GDM; however, mechanistic activity alone does not establish therapeutic suitability during pregnancy. This critical narrative review evaluates selected phytochemicals using a placenta-centered translational perspective that distinguishes evidence from women with GDM, human placental and pregnancy-associated tissues, pregnancy-specific animal models, and mechanistic cellular systems. Human placental studies demonstrate substantial heterogeneity in insulin signaling, mitochondrial function, oxidative stress, inflammation, nutrient transport, and angiogenic regulation, emphasizing that altered pathways may represent pathological, compensatory, or adaptive responses. Among the compounds reviewed, EGCG has the most direct clinical treatment evidence, whereas naringenin shows particularly strong disease-specific placental mechanistic alignment. Quercetin and mangiferin have substantial pregnancy-specific preclinical support, while resveratrol and berberine illustrate how fetal or developmental concerns can limit translation despite promising metabolic effects. Overall, no isolated phytochemical currently combines replicated clinical efficacy, defined maternal–fetal pharmacokinetics, placental relevance, and adequate developmental safety. Future translation requires integrated evaluation across the maternal–placental–fetal unit.