Large volumes of gravy require the preparation of a stock (as you attempted which is awesome, more than many are willing to do!).
Sounds like you used too much water. Here is what I do...
For two quarts of stock, you're going to want about 5lbs of necks/wings/legs. Since I make my stuffing with stock as well (along with turkey soup the next day), I generally make a gallon at a time, which requires 8-10lbs.
Here is my recipe for Brown Turkey Stock...
Brown Turkey Stock
Brown turkey stock is used for hearty poultry or vegetable based dishes. Stripped down, it's essential components (like all good stocks) are clean water (filtered if necessary, but not distilled), bones/joints, aromatics, herbs and spices. It's essential that high quality ingredients are used, as the flavors derived are subsequently reduced and condensed which will magnify any shortcuts taken. The mouth-feel of a good stock is created by collagen in the connective tissues breaking down into gelatin. Browning the bones and aromatics not only brings color, but makes use of the Maillard and Caramelization reactions to increase flavor complexity and depth. It's important that sufficient browning is reached, but care must be taken not to burn anything. Burnt items create a bitter flavor in the stock which is unpleasant and gets worse as the stock is reduced. Some flavors and aromatics are volatile, and care must be taken not to boil them away. Boiling is bad not just for flavor, but also decreases the clarity of the final product. Frequent skimming is also necessary to remove foam and scum which will reduce the stock's quality if left to break down and suspend itself. Stock should be started cold and cooked at a bare simmer. Cooking time begins once the stock reaches a bare simmer.
10-lbs Turkey Bones (or Wings/Drumsticks/Necks)
4-qt + 2-C Water
1 Large Onion - Diced
2 Medium Carrots - Diced
2 Stalks Celery - Diced
6-oz Tomato Paste
2 Medium Cloves Garlic - Crushed
1-t Black Peppercorns
3 Sprigs Fresh Parsley
3 Sprigs Fresh Thyme
2 Bay leaves
Preheat the oven to 425*F and heat a large heavy roasting pan filmed with canola Oil. Add the turkey bones and roast until evenly browned, turning as needed - roughly one hour. Remove the turkey to a large stock pot, and add the water to cover. De-glaze the roasting pan with some water, and add to the stock pot. Bring the turkey to a a bare simmer over medium heat and then reduce the temperature to maintain the bare simmer as necessary. If the water level falls below the level of the turkey, heat some water in a separate sauce pan and gently replenish some of the lost water. Do not completely replenish the lost water, as the stock eventually needs to reduce to 1-gal.
Three and a half hours into simmering, film a skillet with canola oil and add the mirepoix (onion, carrot, and celery). Saute over medium-high heat until the onions are caramelized. Add the tomato paste to the pan and stir constantly until it turns a golden brown and smells sweet. De-glaze the pan with a few ladles of stock, and then add the mixture to the stock pot along with the remaining ingredients.
After the stock has simmered for five hours, carefully strain it with a chinois or cheesecloth and measure the final volume - the target is 1-gal (4-qts). If the volume is short, add enough water to reach 4-qts. If the volume is large, return the strained stock to a simmer, and reduce until 4-qts is achieved. Chill the stock, and then degrease once the fat has stratified and turned solid.
Then for the gravy use the following recipe for "White Sauce" (although it won't be light colored if made with a brown stock as prepared above)...
A traditional French sauce created by thickening a light-colored flavorful liquid with a blond roux. This family of sauces includes Veloute (white poultry/veal/fish stock thickened with roux) and Béchamel (milk thickened with roux). The sauce is usually simmered with aromatics and herbs to fortify it's flavor. It can either be prepared by itself, or incorporated into a recipe using the proper ratios of ingredients.
1-qt Stock or Milk
4-T Clarified Butter
2-oz Onion (Finely Diced)
2-oz Celery (Finely Diced)
1 Large Garlic Clove (Crushed)
1 Fresh Sprig Thyme
1 Fresh Sprig Parsley
1 Bay Leaf
1-t Black Peppercorns
Kosher Salt - To Taste
Freshly Ground White Pepper - To Taste
Heat the clarified butter in a 3-qt saucier over medium heat. Add the onions and celery, and sweat until the onions are transparent and on the verge of taking on color. Add the flour, and stir constantly scraping the bottom until the raw scent of flour has given way to a slightly nutty aroma.
Slowly add the stock, whisking constantly to prevent lumps. Add the remaining ingredients and simmer for 30-45minutes or until the starchy feel of the flour has cooked away.
Strain the sauce through a chinois or rinsed cheesecloth. Season with kosher salt and freshly ground white pepper.
You can then use the stock to prepare a separate pan of "stuffing" (dressing) ahead of time, as well as turkey soup the next day, or turkey pot pie. For the pot pie, use the same gravy recipe but reduce it further until you reach the desired consistency.