[OrionN]

If clowns took food away from your Gigantea, then your Gigantea is not as healthy as he should be.
I would feed him small amount and keep the clown from taken the food. He should improve as you feed him. Feed the clowns first would help. Have a little stick to keep the clown away from the food as you feed him will help too. Also put the food near the mouth and use the stick to help the anemone get food into the mouth. Once the anemone is healthy, nothing will be able to take food away from him.
By small amount, I would consider small pieces up to .25X.25X.25 inch cube.
You can feed every second or third day. Once they are healthy they can eat quite a bit. A 12 inches Gig can down a 1X1X.5 inch piece of fish without problem every few days.