Preheat oven to 400°. Bake pumpkins until just tender, about 20 minutes. Cut off tops and set aside, then scoop out seeds.
In a medium bowl, whisk together heavy cream and whole milk with garlic, parmesan cheese, eggs, salt, and white pepper. Pour mixture into a measuring cup.
Arrange pumpkins in a baking dish large enough to hold them without crowding. Divide custard mixture among pumpkins and cover with tops. Pour enough water into dish to reach halfway up sides. Bake until custard jiggles only slightly, about 30 minutes. Remove from dish and let cool 5 minutes.