Context. Gaia-Sausage-Enceladus is considered the last major merger that contributed to the formation of the Milky Way. Its remnants dominate the nearby accreted stellar halo of the Milky Way. Aims. We aim to characterise the star formation history of Gaia-Sausage-Enceladus through the age and metallicity of its stellar populations. Methods. From Gaia DR3 data, we dynamically defined three Gaia-Sausage-Enceladus samples with different criteria and possible degrees of contamination from other substructures in the halo. Then, we derived the stellar age and metallicity distributions using the CMDft.Gaia package. Results. We identified three main populations of stars and a fourth smaller one following an almost linear age-[M/H] relation. We associate the three oldest populations with the bulk of the star formation that lasted for, at least, ∼ 3−4 Gyr and ended about 10 Gyr ago, its metallicities ranging from −1.7 to −0.8. We categorise these populations into two main epochs: the evolution of GSE in isolation and the merger event. This separation finds independent support from the age-metallicity relation of GSE globular clusters. The fourth population is younger and more metal-rich, at ∼ 8.5 Gyr and [M/H] ∼−0.4; its link to GSE is unclear.