The reason for a space Gravity lab is that we do not know the long term effects of partial gravity and do not know if it is safe for humans to live at partial gravity. If we want to colonize space then we need to understand how safe or unsafe various levels of partial gravity is.
The Gravity Lab was detailed in plans described in 2017.
They want to use two launches of a SpaceX Falcon Heavy or two Blue Origin New Glenn launches. This would launch two pieces of a long gravity lab. The Gravity Lab needs to be sufficiently long for a rotating system for the desired range of partial gravity.