https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-3s/asr903/imc-pim-xe-3s-asr903-book/imc_msdp.pdf

You would configure the glbp 10 priority 200 command on Router C to change the configuration as required.
Gateway Load Balancing Protocol (GLBP) gateway priority determines the role that each GLBP gateway plays and what happens if the AVG fails. In the given scenario, Router A is used as an active virtual gateway. If the AVG in a LAN topology fails, an election process takes place to determine which backup virtual gateway should take over. When you configure this command on Router C, Router C will be elected when Router A fails as an AVG.
Once the configuration change is made, it can be verified by examining the output if the show run command as shown below:

In the above output, it can be determined that the glpb priority 200 command has been applied to the gigabitEthernet0/0 interface on Router
C. If the default priority of 100 had been applied, there would be no line in the output for priority. Because Router B is configured with the default configuration, it will have its priority set to the default level as 100.
You would not use the glbp 10 preempt command on Router B or the glbp 10 preempt command on Router C to change the configuration. You would use this command on a router to enable preemption. Preemption allows a virtual router that was once the AVG to assume its role as active virtual router when it comes back online if it has a higher priority than the current AVG. Alternatively, it can enable a new router with a higher priority to take the role of AVG from the current AVG if the new router has a higher AVG.
You would use not the glbp 10 priority 200 command on Router B to change the configuration. You would run this command if you needed Router B to be elected as the AVR instead of Router C, as running this command on Router B would configure it with higher priority than Router C.
Objective:
Infrastructure Services
Sub-Objective:
Configure and verify first-hop redundancy protocols
References:
Cisco > Home > End-of-Sale and End-of-Life Products > Cisco IOS Software Releases > 12.2T > Product Literature > White Papers > GLBP - Gateway Load Balancing Protocol Cisco > Cisco IOS IP Application Services Command Reference > glbp priority