Gateway Load Balancing Protocol is Cisco proprietary like HSRP, but it adds load balancing. HSRP and VRRP waste standby router capacity. GLBP uses all routers actively.
GLBP elects one router as the Active Virtual Gateway (AVG). The AVG assigns virtual MAC addresses to Active Virtual Forwarders (AVFs). When clients ARP for the virtual IP, the AVG responds with different MAC addresses, spreading traffic across routers.
Up to AVFs can share the load. If one fails, another takes its MAC address.