Melt the butter in a saucepan over medium heat.
Add the flour and cook for 3 minutes.
Whisk in the heavy cream and broth. Bring to a simmer, and cook for 7 minutes.
Reduce the heat to low, and add the gruyere cheese. Cook until the cheese melts.
Add the garlic powder, salt, and pepper. Remove from the heat.
Grease your slow cooker bottom and sides with butter.
Arrange a layer of the potatoes in the bottom of the slow cooker and season with salt and pepper.
Continue layering the potatoes until you have used them all.
Pour the sauce over the potatoes in an even layer.
Cover and cook on high for 4 hours, until the potatoes are tender.
Top with the parmesan and recover until the cheese melts.
Allow to rest for 15 minutes before serving.