Hey,

Gastrulation and PlacentatioN are the process involved in embryo development.

The gastrulation is a process Involving the development of multilayered embryo from single layered fertilized egg . The process of gastrulation give rise to multiple organs and along with that it also helps in establishment of connection between the mother and the developing embryo commonly called as Placentation .

Hence it is clear that gastrulation leads to development of placenta and therefore Placentation occurs after gastrulation I'm the process of embryo development.

Hope that helps.

Thank you.