The most important ones are /dev, /proc, and /sys.

For a UEFI rescue, I recommend this setup (assuming your installed system is mounted at /mnt):

1. Mount virtual filesystems

mount -t proc proc /mnt/proc
mount --rbind /sys /mnt/sys
mount --make-rslave /mnt/sys

mount --rbind /dev /mnt/dev
mount --make-rslave /mnt/dev

mount --bind /dev/pts /mnt/dev/pts

mount --rbind /run /mnt/run
mount --make-rslave /mnt/run

2. Mount UEFI variables

If the rescue USB was booted in UEFI mode:

mount -t efivarfs efivarfs /sys/firmware/efi/efivars

If already mounted, don't mount it again.

Make sure the EFI variables are accessible inside the chroot:

mount --bind \
  /sys/firmware/efi/efivars \
  /mnt/sys/firmware/efi/efivars

Check:

ls /sys/firmware/efi/efivars

3. Enter the installed system

chroot /mnt /bin/bash

4. Verify

Inside the chroot:

mount | grep -E 'proc|sys|dev|run|efivarfs'
efibootmgr -v

Important: The EFI System Partition (ESP), usually FAT32, must also be mounted at the correct location (/boot/efi, /boot, or /efi, depending on your /etc/fstab).

References:

• Debian – GRUB EFI Reinstall

• Debian – Chroot

• ArchWiki – Chroot

One recommendation: If you're rescuing your Gentoo installation, I can give you a complete Gentoo-specific UEFI rescue procedure, including EFI variables, GRUB and kernel/initramfs recovery.