ubuntu 18.04 and above Gem5 version- Version 21.2.1.1 : https://gem5.googlesource.com/public/gem5/+/refs/tags/v21.2.1.1
image file: https://drive.google.com/file/d/1-G29ConJz3a6g13YNm2pbXUZjtRKkVJG/view?usp=sharing
Download the files bootloader,kernel and disk. Set path to full system images using "export".
eg: export M5_PATH=/.../fs_images
build/ARM/gem5.opt configs/example/fs.py --bootloader="$M5_PATH/boot_v2.arm64" --kernel="$M5_PATH/vmlinux.arm64" --disk="$M5_PATH/ubuntu-18.04-arm64-docker.img"