As I understand it, families have their own particular recipe that is handed down. When I first started cooking Indian food, I was learning how to replicate traditional home cookery, though I didn't realise it at that time. I have many memories of adding garam masala at the end of a dish.
As others have said it's not really a BIR thing to use garam masala.. My Butter Chicken recipe does have garam masala in it though I adapted it to BIR from a homestyle recipe.