Well done on taking in an older Grey. It does sound like it has had a bad experience and it will take time love and lots of patience to bring back its trust in humans.
As others have said, even what you think may be mild chastisement like beak tapping will be counter productive. Avoid doing things that you know will cause a biting reaction. Learn to read the signs. It takes time to get your birds confidence and with some birds may have less trust in one human gender compared to another. If you do get a nip try not to react with sudden movements. Most times its is simply letting you know you are being invasive. Be respectful if you want to earn respect.
Don't give your Grey milk because parrots cannot digest lactose. Soya milk or lactose free milk is fine. Never give coffee or alcohol or avocados or chocolate.
In the morning my Misty loves hot water with calcium and vitamin D3 in solution this is good for skin and feather condition.
There is lots on the net about diets for Greys. Much of what you find healthy for yourself is fine for your bird but avoid salt and fatty foods.
Make sure it has a variety of climbing perches and never never clip its wings.
Talk you your bird and sing to it as well. Listen out for its own contact whistle. If you repeat it back to your parrot it will help with the bonding process.
Lots and lots of luck,
Steve n Misty Parrot
Steve n Misty