то ли это или не то?

root #fdisk -l Debian_jessie_mini.img
Disk Debian_jessie_mini.img: 809 MiB, 848297984 bytes, 1656832 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x9c2e57c2

Device Boot Start End Sectors Size Id Type
Debian_jessie_mini.img1 40960 172031 131072 64M b W95 FAT32
Debian_jessie_mini.img2 172032 1656832 1484801 725M 83 Linux
In this case we will copy the image right until the end of the boot partition to the microSD card.

root #dd if=Debian_jessie_mini.img of=/dev/sdb count=172032
Next we will remove the partition table entry for the previous root partition, then create the new root and optionally swap partitions.

root #fdisk /dev/sdb
Welcome to fdisk (util-linux 2.26.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): d
Partition number (1,2, default 2): 2

Partition 2 has been deleted.

Command (m for help): n
Partition type
p primary (1 primary, 0 extended, 3 free)
e extended (container for logical partitions)
Select (default p): p
Partition number (2-4, default 2): 2
First sector (2048-32372735, default 2048): 172032
Last sector, +sectors or +size{K,M,G,T,P} (2048-32372735, default 32372735):
(choose here the size for your root partition, repeat with the swap partition if you want one, then set the appropriate partition types)

Command (m for help): w
Setting up the root partition
We assume that the root partition is /dev/sdb2 and the swap partition is /dev/sdb3.

Create the root filesystem with label "linux" and the swap space.

root #mkfs.ext4 -L linux /dev/sdb2
root #mkswap /dev/sdb3
Install Gentoo to the SD card
Mount the root partition and extract the stage3 tarball and portage snapshot.

root #mkdir /mnt/opipc
root #mount /dev/sdb2 /mnt/opipc
root #tar xfa stage3-armv7a_hardfp-????????.tar.bz2 -C /mnt/opipc
root #tar xfa portage-latest.tar.xz -C /mnt/opipc/usr
If the original Linux image supports multiple boards, mount the boot partition and choose the correct uImage and script.bin for the Orange Pi PC.

root #mount /dev/sdb1 /mnt/opipc/boot
root #cp /mnt/opipc/boot/script.bin.OPI-PC_1080p60 /mnt/opipc/boot/script.bin
root #cp /mnt/opipc/boot/uImage_OPI-2 /mnt/opipc/boot/uImage
Now extract the kernel modules from the image and copy them to your Gentoo system:

root #mkdir /mnt/opipc_image
root #mount -o loop,offset=$((172032*512)) Debian_jessie_mini.img /mnt/opipc_image
root #cp -a /mnt/opipc_image/lib/modules /mnt/opipc/lib
Finish configuration
Edit fstab.

FILE /etc/fstab
/dev/mmcblk0p1 /boot vfat umask=033 1 2
/dev/mmcblk0p2 / ext4 defaults 0 1
/dev/mmcblk0p3 none swap sw 0 0
Set a root password.

root #sed -i "s|root:\*|root:$(openssl passwd -1)|" /mnt/opipc/etc/shadow
Password:
Verifying - Password:
Set compiler and make flags in make.conf.

FILE /etc/portage/make.conf
CFLAGS="-O2 -pipe -march=armv7ve -mcpu=cortex-a7 -mfpu=neon-vfpv4 -mfloat-abi=hard -mtune=cortex-a7"
CXXFLAGS="${CFLAGS}"
MAKEOPTS="-j4"
To work via the debug ttl uart, change:

FILE /etc/inittab
s0:12345:respawn:/sbin/agetty -L 9600 ttyS0 vt100
to