I use the GB on my EXO K4 frame, KUIU bag, and KIFARU belt. However, I do not mount the GB slung low on the rear of the KIFARU belt as suggested by KIFARU, but rather I mount it high on the front of the KIFARU belt. For the attachment at the top of the pack, I use a hunters orange SLIK SLING strap mounted to the pack frame top carry handle. From the one configuration, I can carry rifle barrel vertically in front of my shoulder (not tilted forward as KIFARU suggests.) I can also carry in back of my shoulder, with rifle barrel tilted slightly backward, but still nearly vertical. I can easily mount and dismount the rifle from either position at will while walking with my pack on, nearly as quickly as I can retrieve a rifle on a sling. I can also switch the rifle from front of shoulder to back of shoulder or back again while walking with pack on. This is nearly as quick to deploy the rifle for use as a sling, but much more convenient and secure.
GB position on front of belt, mounted in upper position and adjusted as short as possible.
SLIK SLING strap mounted to carry handle on top of K4 frame. (I use a KUIU bag, on a K4 frame, with a KIFARU belt.)
SLIK SLING strap post mounted on shoulder strap.
Front of shoulder carry. (Notice the rifle barrel is behind my ear, out of view, and largely out of the way, though not as out of the way as rear shoulder carry.)
Back of shoulder carry.
Carried in front or back of shoulder this way, the rifle barrel does not impede field of vision, or ease of walking or climbing. Dismounting rifle for use is VERY fast and silent from front of shoulder carry, still silent and nearly as fast from back of shoulder carry, and completely out of the way in back of shoulder carry for hand over hand climbing situations, and very quickly and easily switched back and forth as needed. I can stalk low on all fours with rifle in either position.