I have a few different ways that I make a veggie gravy. One is to fry onions in a pan, add marigold veg bouillon (dissolved in water), a tiny bit of marmite and a bay leaf, simmer 10 minutes or until the stock has reduced, add cornflour to thicken.
Another way I use a combination of marigold veg bouillion & kallo garlic stock. Or you could use veg stock & red wine.