For starting out, the GR1 is a great bag to get used to rucking with. While it does not have the ruck plate pocket like the Rucker, it has an elastic sleeve that you can put a ruck plate in which holds it in place. For casual rucking or going a few miles regularly, this is all you need.
However, if you plan on using your gear to ruck longer distances or add some PT to your rucking routine, you will want to go with the Rucker instead as the plate pocket design for those is made to keep your ruck plate from shifting during those movements.