We all have our own favourite gravy recipes and I will include a few below, but the one single thing which will improve your gravy is by using real home made stock. Personally I would make a chicken stock to go with Turkey as cheap chicken pieces are easy to come by and because turkey stock can be a bit rich which some guests and especially children might find a little strong.
Roast the chicken wings until golden brown in a hot oven. Put them and all juices into a big pan. Cover with water and bring to the boil then reduce to a very low simmer. Cook for 3 hours, topping up the water level with hot boiled water if needed. Now add the vegetables and bring back to the boil then reduce again to a very low simmer and cook for 1 more hour. Strain into a smaller cooking pot through a fine sieve or muslin cloth. You want about 1 pint of stock so if you have more put it on the boil to reduce down. Leave to cool and then freeze or refrigerate. Frozen will be good for 1 month, refrigerated will last for 3-4 days.
If you made the stock yourself, you will have a layer of chicken fat on the top. Scrape this off and fry the onion on a low heat until golden brown. Add the garlic after crushing and cook for a further 2-3 minutes. Add the stock, mustard and wine and puree with a hand blender until smooth. Now heat a large knob of butter in a small pan and add the flour to brown slightly, you should end up with a thick paste. Add this a little at a time to the gravy in order to thicken it, blend with the hand blender to work out any lumps. The butter will add a slight gloss to the gravy too. Finally taste and adjust the gravy with salt and pepper to your taste. If you have just roasted your turkey and removed it from the baking pan then I would save the wine to put in the pan and scrape up all the bits on the bottom and all the juices in the pan, pour in with the rest of the gravy and blend up, this will really max up the flavour.
Cut the meat into very small pieces, slice the onion and carrot and put them in a small saucepan with the butter. Keep stirring over a hot flame until they have coloured a little. Add the water and the rest of the ingredients and simmer for half an hour. Skim well, strain and season.