Aims: To describe the metabolic phenotypes of early gestational diabetes mellitus and their association with adverse pregnancy outcomes. Methods: We performed a post hoc analysis using data from the Vitamin D And Lifestyle Intervention for gestational diabetes prevention (DALI) trial conducted across nine European countries (2012–2014). In women with a BMI ≥29 kg/m², insulin resistance and secretion were estimated from the oral glucose tolerance test values performed before 20 weeks, using homeostatic model assessment of insulin resistance and Stumvoll first-phase indices, respectively. Women with early gestational diabetes, defined by the International Association of Diabetes and Pregnancy Study Groups criteria, were classified into three groups: GDM-R (above-median insulin resistance alone), GDM-S (below-median insulin secretion alone), and GDM-B (combination of both) and the few remaining women were excluded. Results: Compared with women in the normal glucose tolerance group (n = 651), women in the GDM-R group (n = 143) had higher fasting and post-load glucose values and insulin levels, with a greater risk of having large-for-gestational age babies [adjusted odds ratio 3.30 (95% CI 1.50–7.50)] and caesarean section [adjusted odds ratio 2.30 (95% CI 1.20–4.40)]. Women in the GDM-S (n = 37) and GDM-B (n = 56) groups had comparable pregnancy outcomes with those in the normal glucose tolerance group. Conclusions: In overweight and obese women with early gestational diabetes, higher degree of insulin resistance alone was more likely to be associated with adverse pregnancy outcomes than lower insulin secretion alone or a combination of both.