I just use normal bisto and syn it. I used to be scared if syns and use syn free everything, but actually, that's silly. For me, it was encouraging spending my syns on treats every evening instead. Which didn't seem a good habit!
I have heard that the SW Iceland gravy is nice, but have yet to find it in my local store. I use Bisto gravy granules, and just count the syns: 4 tbsp gravy when made up as directed is 1 syn. Sometimes, I fry sliced onions in Frylight and stir the gravy through that. Agree with honeysucklejasmine that there's no need to use everything syn-free: I like using my syns to enhance my enjoyment of a meal and not feel deprived.
Some in our group have used mushy peas to make into a thick gravy. I think they use oxo cubes for the meaty flavour then blend the mushy peas with water to thin. Haven't tried it myself to but make mushy pea curry sometimes so I guess it's similar to a degree x