Gentoo provides a Github mirror of portage that can be used as an alternative to rsync. The primary resource for how to do this is Gentoo Wiki : Portage/Sync. It might seem daunting but its quite straight-forward.
Modify /etc/portage/repos.conf/gentoo.conf to have the following (note I was cautious and lazy and simply commented out the existing settings)…
[gentoo] location = /usr/portage # sync-type = rsync # sync-uri = rsync://rsync.gentoo.org/gentoo-portage sync-type = git sync-uri = https://github.com/gentoo-mirror/gentoo.git auto-sync = yes priority = 1000
Now as root remove your existing portage tree and then sync it anew (optionally make a backup if you are concerned).
rm /usr/portage/distfiles/* tar cjvf /tmp/portage.tar.bz2 /usr/portage/. rm -rf /usr/portage/* emerge --sync
Thats it you're updated.
You might have your own local overlay in say /usr/local/portage/ and overlays from layman which will be referenced in /etc/portage/make.conf as…
# Overlays PORTDIR_OVERLAY="/usr/portage/local" source /var/lib/layman/make.conf
If you have your /usr/portage/local/ under Git version control (and if not why haven't you) then you can also configure portage to sync it from a backup on GitHub/GitLab
[My-Overlay] location = /usr/local/portage sync-type = git sync-uri = git://gitlab.com/my/overlay.git auto-sync = no