How long does gravy last in the fridge? I made it Sunday and forgot to freeze it -- should I toss by Thurs?

It'll be fine; reheat gently, and have some warm liquid (stock, milk, water) ready, you may need to thin it a bit

3-4 days is a safe window for using up gravy, but definitely give it a smell before you reheat it. If it smells off, I'd recommend making a new batch. If it smells fine, it should be good to reheat.