Unfortunately it is typical of a gigantea that isn't going to make it.
IME, a healthy gigantea rarely deflates much.
Once they start to deflate, they tend to do so more and more often until they stop inflating at all and then they start to decompose. This process may take a couple months or a week.
Some have had luck reversing this process by increasing the flow and placing small foods like mysis directly in the mouth.