It’s possible, but why would you? It’s just a normal Gentoo install, but worse. This is Diogenes holding up a de-feathered chicken and claiming “behold, a man!”.

Not much to say here, just download and untar a stage3 in, say, a directory under the Termux home directory, then:

mount --bind /dev gentoo/dev   # your phone might not have devtmpfs
mount -t devpts devpts gentoo/dev/pts
mount -t proc proc gentoo/proc
mount -t sysfs sys gentoo/sys
mount -t tmpfs shm gentoo/dev/shm

chroot, probably using the following command to clear the env (derived from Chapter 7.4 of LFS 12.1):

sudo chroot "gentoo/" /usr/bin/env -i   \
    HOME=/root                  \
    TERM="$TERM"                \
    PATH=/usr/bin:/usr/sbin     \
    /bin/bash --login

Your kernel will likely be lacking in namespace support; ls /proc/self/ns to see what you have. This annoys emerge, which does sandboxing using namespaces.

In this case, use aggressively insecure FEATURES to appease emerge in make.conf:

FEATURES="-sandbox -ipc-sandbox -network-sandbox -pid-sandbox -usersandbox -userfetch"