The padded hip belt is compatible with rucks that feature side MOLLE, including the Rucker 4.0, GR1, and GR2. It is not compatible with the Bullet, GR3, or RPC.
The padded hip belt can be easily attached or removed using the MOLLE system. To do this, locate and pull out the MOLLE strap—while it may appear sewn in at first, be sure to check both sides of the attachment point.
Below is a screenshot showing where to find the MOLLE strap, and you can also follow this video for step-by-step instructions: Hip Belt Video