How to extract rootfs from rootfs.gz

[carrie17]

Hello,I use FGT_VM64-v7.4.2.F-build2571 for the experiment and mount it vmdk with ubuntu22.04. I got Fortios already and rootfs.gz in it. but it is not gzip file.

$ file rootfs.gz
rootfs.gz: data
$ binwalk rootfs.gz
DECIMAL       HEXADECIMAL     DESCRIPTION
--------------------------------------------------------------------------------
20517347      0x13911E3       Cisco IOS microcode, for "F"
55687191      0x351B817       MySQL MISAM compressed data file Version 8

Would you like to ask how to extract the file? thanks!