The best way to do it is to create a roux and a really intense broth. Make the roux by whisk together equal parts fat (either butter or oil) and flour... i'd say 1/4 cup of each. Stir CONSTANTLY for 3-5 minutes until you have a smooth, golden paste. You want to make sure you cook that "flour" taste out of it. To make the intense broth, if you're going vegetarian, boil 2 cups vegetable broth with 2 teaspoons of granulated vegetable boullion. Adding boullion to the broth mimics the flavor intensity that the drippings would've provided. If you can, get some fresh herbs like sage, thyme, and rosemary, and boil the whole branches of it in the broth. I'd boil it for 10 minutes or so, and add a little garlic powder and black pepper. Probably won't need salt due to the boullion and broth. Remove the twigs from the broth and slowly add it to your roux, whisking constantly until smooth. Leave it on low heat for another 3 mintues or so, still stirring it alot, until it has thickened. That should do it. Of course if you're not worried about vegetarians, you can use chicken broth and chicken boullion.
EDIT: You want to cook that roux on the stove at medium heat, and I'd use butter instead of oil because the flavor is much better. You stir it constantly because if it burns, you're screwed and it will make everything it touches taste terrible. Powdered vegetable boullion is not easy to find, but its out there. And adding a splash of white wine to it would definitely help.
Sorry... I'm a food obsessive. Another way to do it, if you're not into simple is to sautee some onion, celery, garlic, and those fresh herbs in your 1/4 cup of butter. Remove those once they've been sweated, then add your flour and make your roux.
There are is only two components to a gravy. A rich stock and a thickener. For a rich stock without drippings, roast some mirepoix (2 parts onions, 1 part celery, one part carrots), garlic and turkey trimmings in the oven at 400 degrees for 45 minutes, stirring every ten minutes or so, until brown. Add chicken or turkey stock/broth to it and scrape up all the brown bit off the bottom. Add some white wine and thyme. For the thickener, you can use a roux which is just fat/oil with equal parts flour or corn starch and water. BTW, to make giblet gravy, just make the stock out of giblets and the turkey neck.
^^^ This... The only difference for me is, I only do a roux with butter and flour. Never made it with water before.
For a healthier alternative, you might want to consider this cranberry dipping sauce , almost like a thanksgiving version of BBQ sauce or a cranberry gravy. I made some to try out before Thursday, it's delicious and simple to make, and you can also put it on ice cream. <object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=9,0,124,0" width="400" height="323"><param name="movie" value="http://common.scrippsnetworks.com/common/snap/snap-2.1.21-embed.swf?channelurl=http://www.foodnetwork.com/food/channel/xml/0,,691-VIDEO,00.xml&channel=691"/><param name="allowFullScreen" value="true"/><param name="allowscriptaccess" value="always"/><param name="bgcolor" value="#FFFFFF"/><embed src="http://common.scrippsnetworks.com/common/snap/snap-2.1.21-embed.swf?channelurl=http://www.foodnetwork.com/food/channel/xml/0,,691-VIDEO,00.xml&channel=691" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" bgcolor="#FFFFFF" width="400" height="323"/></object>
turkey broth from your just cooked turkey, flour, salt and pepper...maybe a little bit of water and butter, but that's it... mix in the flour a little at a time and keep whisking until thick...
