Full steps are at https://wiki.installgentoo.com/wiki/VirtualBox
Steps for booting from the ISO image
https://www.onlinecomputertips.com/support-categories/software/662-virtualbox-boot-iso
Partition creation step
https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Disks#GUID_Partition_Table_.28GPT.29
Installation steps video
https://www.youtube.com/watch?v=q9_sXkA4Rv8