Gestational weight gain (GWG) is associated with postpartum weight retention (PPWR) in women. The strength of the association between GWG and long-term PPWR and body mass index (BMI), however, is still unclear. Publications from different databases were systematically extracted and the articles relevant to this study were reviewed to quantify the effect estimate of GWG on PPWR and BMI using a bias-adjusted method. The Institute of Medicine categories of “inadequate,” “adequate,” and “excess” were used to define GWG. The time span for PPWR was divided into three periods (<1 year, 1 year to 9 years, and ≥15 years) to determine outcome at different times postpartum. Twelve studies met the eligibility criteria and were included in the analyses. Women with an inadequate GWG had a significantly lower mean PPWR of −2.14 kg (95%CI, −2.61 to −1.66) than women with an adequate GWG, who had a mean PPWR of 3.15 kg (95%CI, 2.47 to 3.82) up to 21 years postpartum. Over the postpartum time span, a U-shaped relationship was observed between the weighted mean difference calculated for women with excess GWG and the weighted mean difference calculated for women with adequate GWG, and this relationship was time independent between these two groups. Postpartum BMI showed a similar relationship and magnitude of change, but the exact loss or gain was difficult to assess due to fewer studies (n = 5) with considerable heterogeneity of BMI measurements. The findings of this study suggest that GWG outside of the Institute of Medicine recommendations can lead to both short-term and long-term postpartum weight imbalance.