I’m making one pan (oven baked) chicken and rice...I want white gravy smothered all over it. Would…

I’m making one pan (oven baked) chicken and rice...I want white gravy smothered all over it. Would you put it in when you put the uncooked chicken and rice in? Or make gravy in a pan and just pour it on top?

Recipe calls for a can of cream of mushroom would I sub white gravy or add both? Lol I suck at cooking this is my first attempt at chicken and rice but I know my grandma always used to make it in one pan just can’t remember how she did the gravy?