GLBP is also Cisco Proprietary just like HSRP, the difference between GLBP and HSRP/VRRP is that it adds load balancing features in addition to the router redundancy. Hosts will still point to a default gateway IP address, but GLBP will cause that traffic to be forwarded to one of up to four routers i na GLBP group.
GLBP has an Active Virtual Gateway (AVG) that assignes each router in the GLBP group a virtual MAC address, using the format: 0007.b400.XXYY where XX is the GLBP group and YY is a different number for each router (01,02,03 or 04)
When a client ARPs for the virtual IP address of the default gateway, GLBP AVG replies with one of the four possible routers in the group with their Virtual MAC address. By doing so in a round robin fashion host traffic is load balanced across all four routers .