Totipotent stem cells: can differentiate into all cell types in body and placenta
Pluripotent stem cells: can differentiate all the cell types in the body (aka embryonic stem cells)
Multipotent stem cells: can differentiate into only a couple of cell types (aka adult stem cells)
At the beginning of development we need stem cells that are able to differentiate into all different cell types, but once a certain stage of development is reached, this isn't necessary anymore, (therefore adult/multipotent stem cells only differentiate into a couple of cell types).
Spoiler
If the cells of the gastrula are considered adult stem cells it would be because they can differentiate into multiple (but not all) cell types