Vegetable Stock Cube
Showing my origins here, I use Vegetable Oxo cubes. They are very salty and loaded with umami response-inducing ingredients like monosodium glutamate and disodium 5 ribonucleotides. Adds real flavour to any vegan mix.
If you are unwilling to use something that intentionally adds monosodium glutamate, feel free to liberally sprinkle in soy sauce, which has monosodium glutamate in it, but produced naturally during the fermentation.
You can of course replace the Vegetable Oxo cubes with any other vegetable bouillon stock cubes of your choice.
If using gravy powder, beware any thickener it may have in there, for example starch.