The two ways are to make a nicely flavoured thing and thicken it with cornflour at the end, or you can make a velouté with a bit of wine and stock. My recommendation is the former because you need a lot of fat for velouté.
Chicken GravyStart with a Panch of just fennel seeds, then add a battuto of garlic and sage then mirepoix and when that’s all lovely, put a glass of white wine in. Bring that to the boil to get rid of the alcohol and then add chicken stock from a cube. Simmer it for while so the flavours mingle then blend it. I’d then pass it through a sieve and then thicken it with cornflour
Beef gravyFollow chicken gravy but substitute the fennel in the Panch for cumin and if you’re feeling brave replace the sage with ginger in the battuto (try it, it really works). Red wine instead of white, beef stock instead of chicken and add a spoon of mustard – I like French but you choose.
Lamb GravyAgain, add dried mint at the battuto stage and you can’t go wrong with rosemary. Like the beef version I’d add mustard but it’s not important and a spoon of mint sauce is cool.
Pork GravyYou don’t really get pork cubes but there are bacon ones. I’d stick with the fennel and sage and use cider instead of the wine and of course mustard just goes with pork so go for it. Chicken stock is fine too.