Yes, gravy is halal if it is prepared from halal meat and ingredients. For gravy to be considered halal, the meat used in its preparation must come from animals slaughtered according to Islamic law, and all other ingredients must also be halal. If the gravy is made with haram meat or other non-halal ingredients, it would not be considered halal.

Sources:

• My Halal Restaurants
• IslamQA

And Allah knows best.