Multifetal pregnancies are at risk of adverse maternal, neonatal and long-term health outcomes, and gestational weight gain (GWG) is a potentially modifiable risk factor for several of these. However, studies assessing the associations of GWG with long-term health in twins are rare, and studies which do assess these associations in twins often do not account for gestational age. Since longer gestations are likely to lead to larger GWG and lower risk of adverse outcomes, adjusting for gestational age is necessary to better understand the association of GWG with twin health outcomes. We aimed to explore long-term associations of GWG-for-gestational-age with twin anthropometric measures. The Peri/Postnatal Epigenetic Twins Study (PETS) is a prospective cohort study, which recruited women pregnant with twins from 2007 to 2009. Twins were followed-up at 18 months and 6 years of age. GWG-for-gestational-age z-scores were calculated from pre-pregnancy weight and weight at delivery. We fitted regression models to assess associations of GWG with twin weight, height and BMI at birth, 18 months, and 6 years. Of the 250 women in the PETS, 172 had GWG measured throughout pregnancy. Overall, higher GWG-for-gestational-age z-scores were associated with higher birthweight (β: 0.32 z-scores, 95% Confidence Interval (95% CI): 0.19, 0.45), BMI (β: 0.29 z-scores, 95% CI: 0.14, 0.43) and length (β: 0.27 z-scores, 95% CI: 0.09, 0.45). However, these associations were not observed at 18 months or 6 years of age. GWG was associated with twin length, weight and BMI at birth but not during childhood. Further research is needed to determine the long-term effects of GWG on twin health outcomes.