Hello Walter
In the case where you have many subnets, you would have multiple SVIs within each of the L3 switches, acting as the default gateways. For each one of these SVIs you would configure gateway redundancy such that you create a virtual gateway for each subnet/VLAN. Take a look at the following diagram:
Here you have 10 VLANs/subnets all of which have SVIs in both devices. You configure 10 virtual gateways, one for each VLAN/subnet. In such a scenario, you would typically make one of the switches the active for half of the VLANs, and the other switch active for the other half. This way, the traffic load can be shared across the switches. This is especially useful with HSRP which does not perform automatic load balancing.
In the above diagram, SW1 is the active gateway for VLANs 10 to 50, while SW2 is the active gateway for VLANS 60 to 100. In the event that one switch fails, the other will take over for all VLANs.
For GLBP, this separation of VLANs is not necessary as it can perform load balancing automatically across both switches for each individual VLAN.
In order to understand this more fully, I suggest you perform the labs in the following lessons, which should clarify this mechanism.
I hope this has been helpful!
Laz