CONFIG_MAXPHYSMEM_1GB is not available for thedefaultarchitecture x86.
Result is shown for architecture riscv

1GiB

configname: CONFIG_MAXPHYSMEM_1GB

Maximum Physical Memory
└─>1GiB
In linux kernel since version 5.10.2 (release Date: 2020-12-21)  
depends
CONFIG_32BIT