Final answer:
The priority nursing action is to administer the prescribed magnesium sulfate.
Step-by-step explanation:
The priority nursing action when a pregnant patient with severe gestational hypertension is admitted to the healthcare facility is to administer the prescribed magnesium sulfate. Gestational hypertension is a serious condition that can lead to complications for both the mother and the baby. Magnesium sulfate is often used to prevent seizures associated with severe hypertension during pregnancy, a condition known as preeclampsia.