Hanging Gardens B is a natural environment module on the Covenant holy city High Charity. This area, like Hanging Gardens A, is very complicated. There are two hills, with only one way to get to them. It can be extremely hard to find.
During the first hours of the Great Schism, the Swords of Sanghelios invaded, but were attacked by Brutes of the Covenant Loyalists. All the Separatist Elites were slaughtered in the heated combat. John-117 arrived just in time to see two berserking Brutes massacre the retreating Grunts. John-117 slew them all in the events of the campaign level Gravemind.
It can be assumed that the Hanging Gardens were destroyed when the Flood invaded High Charity, or if not, it is almost certain that they were destroyed along with the rest of High Charity at the end of the Human-Covenant war.