Obvoiusly, it’s not the Antiphonale and Graduale Romanum (which between the two would contain all the chants needed for Office and Mass), nor is it even the Liber Usualis (which is a compilation from the other two), but it does have chants for Sundays and Feastdays adapted to the Ordinary Form of the Roman Rite.
So no, the Gregorian Missal does NOT contain all the corpus of Gregorian Chant, even for mass.