I know calling it gravy seems strange. I grew up calling it that. I think it is more of a North East Italian American thing.
Not to far of a stretch though. You think of gravy as a flour roux type sauce that is made from bone stock or drippings. Real tomato gravy like mine is made with lots of bones. Basically a tomato bone stock.
Marinara sauce is never called gravy. Marinara sauce is called tomato sauce. They look the same, both made with tomatoes. Just that the gravy is made with lots of bones and meat.
My Grandmother always made gravy when she made a Ragu. That is a dish that is a mish mash of meat (maybe even left overs) that is mixed into a tomato gravy. with fried peppers and veg served on rigatoni. Man I loved that. Only time we got to eat well growing up, both my mom and my dad couldnt cook to save their lives.
I make the tomato gravy ideally with some pork neck bones or pigs feet, lamb neck bones, beef marrow bones. All cheap, you can use any one or combination.
For the "Gravy":
Brown the bones in some oil in a big stock pot. After brown throw in a head or two of garlic peeled and chopped. Throw in a half a small onion chopped. a healthy pinch of salt, fresh cracked pepper (lots). dried Oregano, dried basil, grate in a whole head of nutmeg. a pinch of red pepper seed. Ideally fresh but dried ok of bay leaf, rosemary. De-glaze with a small amount of red wine or water. Then add a couple of large cans Tomato puree and crushed. Add some water till it is kinda thin but not watery. Bring to a boil then turn down to a simmer. Add a teaspoon or two of sugar (sort of breaks the acid in the tomatoes). Add a small can of tomato paste. Grate in a fist full of locatelli cheese or other type of parm.
Cover and simmer till the meat is falling off of the bones.
Take the bones out shaking off the excess gravy into the pot. Pick off any meat you can find chop it and return it to the gravy.
There are variations but that is traditional Italian tomato gravy.
Ill post my lasagna recipe later, gotta go for now.
Its interesting though ill be honest. Instead of using ricotta cheese witch is watery cheese made from dairy whey
I use my home made paneer, It is basically a firmer milk ricotta that cakes and holds together really well. When you slice the lasagna it stays formed and comes out in a nice layered slice.