Yes it's okay to use expired gravy mix.
I use expired gravy mix that comes in packages all the time.
The expiration date on the gravy mix just means that it is not as fresh as before and may not be as tasty to you.
However the expiration date on the gravy does not mean that it's actually bad or unsafe to eat.
There's a store here that sells expired groceries that are still safe to eat and the price of the food is half the price of fresh food.
I buy some of the expired food and it's just as good as fresh food.
I buy a lot of that expired gravy mix and use it all the time and to me it tastes just as good as the fresher gravy mix.