Cut the pumpkins vertically into 3/4 inch slices. Remove the seeds and stringy parts. Brush both sides of each slice liberally with olive oil. Sprinkle with sea salt and rosemary.
Place the slices on the grill for about 5 minutes on each side or until dark grill marks appear. Turn and grill the other side until you can easily pierce the pumpkin slice with a fork. Make sure that the pumpkin is tender.