I've only managed gravy this great twice, so I'm writing it down:
The day before
Roast some turkey parts in the oven at 350 degrees until they are caramelized. If you have spatchcocked your turkey, these parts can be the backbone and neck. If you have not, you can purchase a package of wings or drumsticks and use those.
After you've roasted the pieces, deglaze the pan with a cup of water, scraping all those wonderful roasting bits from the pan. Pour all the liquid into a container and store in the fridge to use the next day.
Take the pieces you've roasted, and simmer them for a couple hours in 6-8 cups of water. Store that stock you made in the fridge for the next day. Do not pour the stock into the pan drippings you made. Use a separate container.
On gravy day
Melt a stick of butter in a saucepan and whisk in 3/4 cup flour. Turn it down low and cook this (yes, it's a roux) for 4-5 minutes. Keep stirring it during the cooking time to prevent scorching. Don't allow it to get very brown...just a dark golden color.
Get the pan drippings and the stock containers from the fridge. Skim off the fat layer from the top of the pan drippings and discard. Whisk the remaining pan drippings into your roux. Now start whisking in some of the stock in 1/2 cup increments. You most likely will use 3 cups of the stock, but add it slowly. You want to monitor the thickness of your gravy so you get exactly what you want. As you add 1/2 cup, it'll seem watery and then tighten up with a few more seconds of cooking as it absorbs the flour from the roux. Once you get it the thickness you want, adjust the salt and pepper. Let it simmer for a few minutes.
I made my gravy about 3 hours before dinner. So poured it into my very small slow cooker and let it wait there on warm. It didn't scorch and didn't require any real attention. But you'll want to check it periodically to be sure your slow cooker isn't too hot.