GLBP allows a set of routers to have ARP resolutions loabalanced across the devices, thereby spreading the work around to each of the devices.This only really works (SLB aside) if you have many gw's feeding the GLBP'd cluster. The reason SLB won't work is that for SLB connections uplink and downlink packets need to go through the same device and GLBP has no way of enforcing this. The SLB config that can work with GLBP is per-packet loadbalancing where SLB keeps no state and can handle asymmetric responses. Typically GLBP is good at a layer2-to-layer3 edge where there are lots of common devices; that means mostly end stations, sometimes simple server farms. When more sophisticated load balancing is desired, SLB, CSM, etc. is the answer.