I haven’t made the cabbage schnitzel but I’ve done cauliflower wings before which is a similar preparation. They’re not at all comparable to a real chicken wing but they’re good in their own right. I tend to use the mock meats if I’m trying to recreate something meaty.
I’m sure there are a bazillion vegan gravy recipes online for you to pick from, probably using ingredients like mushrooms, soy sauce, tamari, liquid aminos, nutritional yeast, or bouillon for the umami flavor that you would normally get from meat. The base I use for white gravy is just vegan butter, flour, and almond milk — I don’t think that one would have meat in it anyway. Or, going the lazy route and buying it premade is an option, too.
Sorry, I don’t have a favorite gravy recipe to offer or anything, I just make whatever recipe goes with whatever dish I’m making at the time. How they compare to a meat-based gravy? I don’t know, I’m just now realizing that I don’t think I’ve ever had “real” gravy. I didn’t add it to food when I was a kid and I’d eliminated meat from my diet by the time I was an adult.