i wouldnt give gravy to a 6 month old baby
simply because until they were actually 6 months i would only give them fruit / veg
then at 6 months id move over to dairy.... so start incoperting cheese and yogurt and white sauce etc, and bread and pasta
then about 6.5 months id give chicken
then a week or so later id give fish etc...
so baby would be at least 7+ months before id even THINK about it!
I wouldnt add gravy to their food at all for a while (u can make simply veg stock full fo flavour minus the salt) or just use veg water, its FINE!
however by the time i was moving onto bolognese sauce etc (for example whizzing up some bol sauce that another child / adult was having) i wouldnt worry about it.
Im a great believer in no food at all except fruit and veg purees and babyrice until 6 months, and there are lots of food to try before gravy IUSWIM!