/var/db/pkg
From Gentoo Wiki
Stores information about installed packages. One of the Gentoo folders that you should backup (git to a remote origin is a good option).
How it looks like:
user $tree /var/db/pkg/var/db/pkg ├── acct-group │ ├── 1password-0 │ │ ├── 1password-0.ebuild │ │ ├── BUILD_TIME │ │ ├── CATEGORY │ │ ├── CBUILD │ │ ├── CFLAGS │ │ ├── CHOST │ │ ├── CONTENTS │ │ ├── COUNTER │ │ ├── CXXFLAGS │ │ ├── DEFINED_PHASES │ │ ├── DESCRIPTION │ │ ├── EAPI │ │ ├── FEATURES │ │ ├── INHERITED │ │ ├── IUSE │ │ ├── IUSE_EFFECTIVE │ │ ├── KEYWORDS │ │ ├── LDFLAGS │ │ ├── PF │ │ ├── SIZE │ │ ├── SLOT │ │ ├── USE │ │ ├── environment.bz2 │ │ └── repository │ ├── apache-0-r3 │ │ ├── BUILD_TIME │ │ ├── CATEGORY │ │ ├── CBUILD │ │ ├── CFLAGS │ │ ├── CHOST │ │ ├── CONTENTS │ │ ├── COUNTER │ │ ├── CXXFLAGS │ │ ├── DEFINED_PHASES │ │ ├── DESCRIPTION │ │ ├── EAPI │ │ ├── FEATURES │ │ ├── INHERITED │ │ ├── IUSE │ │ ├── IUSE_EFFECTIVE │ │ ├── KEYWORDS │ │ ├── LDFLAGS │ │ ├── PF │ │ ├── SIZE │ │ ├── SLOT │ │ ├── USE │ │ ├── apache-0-r3.ebuild │ │ ├── environment.bz2 │ │ └── repository │ ├── audio-0-r3 │ │ ├── BUILD_TIME │ │ ├── CATEGORY │ │ ├── CBUILD │ │ ├── CFLAGS │ │ ├── CHOST │ │ ├── CONTENTS │ │ ├── COUNTER │ │ ├── CXXFLAGS │ │ ├── DEFINED_PHASES │ │ ├── DESCRIPTION │ │ ├── EAPI │ │ ├── FEATURES │ │ ├── INHERITED │ │ ├── IUSE │ │ ├── IUSE_EFFECTIVE │ │ ├── KEYWORDS │ │ ├── LDFLAGS │ │ ├── PF │ │ ├── SIZE │ │ ├── SLOT │ │ ├── USE │ │ ├── audio-0-r3.ebuild │ │ ├── environment.bz2 │ │ └── repository │ ├── avahi-0-r3 │ │ ├── BUILD_TIME │ │ ├── CATEGORY │ │ ├── CBUILD │ │ ├── CFLAGS │ │ ├── CHOST │ │ ├── CONTENTS │ │ ├── COUNTER │ │ ├── CXXFLAGS │ │ ├── DEFINED_PHASES │ │ ├── DESCRIPTION │ │ ├── EAPI │ │ ├── FEATURES │ │ ├── INHERITED │ │ ├── IUSE │ │ ├── IUSE_EFFECTIVE │ │ ├── KEYWORDS │ │ ├── LDFLAGS │ │ ├── PF │ │ ├── SIZE │ │ ├── SLOT │ │ ├── USE │ │ ├── avahi-0-r3.ebuild │ │ ├── environment.bz2 │ │ └── repository │ ├── cdrom-0-r3 │ │ ├── BUILD_TIME │ │ ├── CATEGORY │ │ ├── CBUILD │ │ ├── CFLAGS │ │ ├── CHOST │ │ ├── CONTENTS │ │ ├── COUNTER │ │ ├── CXXFLAGS │ │ ├── DEFINED_PHASES │ │ ├── DESCRIPTION │ │ ├── EAPI │ │ ├── FEATURES │ │ ├── INHERITED │ │ ├── IUSE │ │ ├── IUSE_EFFECTIVE │ │ ├── KEYWORDS │ │ ├── LDFLAGS │ │ ├── PF │ │ ├── SIZE │ │ ├── SLOT │ │ ├── USE │ │ ├── cdrom-0-r3.ebuild │ │ ├── environment.bz2 │ │ └── repository ...
user $du -d 1 -h /var/db/pkg2.3M /var/db/pkg/app-admin 164K /var/db/pkg/dev-tcltk 124K /var/db/pkg/app-dicts 2.0M /var/db/pkg/media-sound 604K /var/db/pkg/x11-base 1020K /var/db/pkg/app-portage 132K /var/db/pkg/platform 1.1M /var/db/pkg/app-editors 2.0M /var/db/pkg/sys-fs 224K /var/db/pkg/www-servers 772K /var/db/pkg/dev-vcs 3.8M /var/db/pkg/sys-devel 260K /var/db/pkg/gnome-extra 68M /var/db/pkg/.git 6.6M /var/db/pkg/dev-lang 320K /var/db/pkg/app-accessibility 144K /var/db/pkg/lxde-base 840K /var/db/pkg/gnome-base 1.8M /var/db/pkg/gui-libs 260K /var/db/pkg/net-print 1.7M /var/db/pkg/app-benchmarks 1.1M /var/db/pkg/sys-process 304K /var/db/pkg/app-pda 2.2M /var/db/pkg/app-office 256K /var/db/pkg/sci-geosciences 2.2M /var/db/pkg/app-crypt 164K /var/db/pkg/mail-mta 1.5M /var/db/pkg/media-fonts 7.4M /var/db/pkg/x11-themes 5.1M /var/db/pkg/dev-qt 3.3M /var/db/pkg/dev-cpp 280K /var/db/pkg/app-i18n 3.5M /var/db/pkg/net-libs 1.6M /var/db/pkg/acct-user 156K /var/db/pkg/games-engines 936K /var/db/pkg/dev-db 824K /var/db/pkg/gui-apps 360K /var/db/pkg/net-p2p 524K /var/db/pkg/dev-java 8.5M /var/db/pkg/x11-libs 148K /var/db/pkg/games-emulation 680K /var/db/pkg/x11-terms 480K /var/db/pkg/x11-drivers 4.2M /var/db/pkg/media-video 3.5M /var/db/pkg/net-misc 3.7M /var/db/pkg/sys-libs 2.7M /var/db/pkg/media-plugins 116K /var/db/pkg/sec-keys 144K /var/db/pkg/games-util 1.4M /var/db/pkg/net-im 2.6M /var/db/pkg/dev-build 1.5M /var/db/pkg/app-eselect 6.8M /var/db/pkg/app-text 860K /var/db/pkg/sys-auth 13M /var/db/pkg/app-misc 160K /var/db/pkg/dev-go 164K /var/db/pkg/app-doc 3.1M /var/db/pkg/x11-misc 8.5M /var/db/pkg/virtual 192K /var/db/pkg/net-irc 256K /var/db/pkg/mpv-plugin 160K /var/db/pkg/sys-block 1.2M /var/db/pkg/net-analyzer 20M /var/db/pkg/sys-kernel 1.2M /var/db/pkg/app-containers 132K /var/db/pkg/app-vim 1.5M /var/db/pkg/app-shells 252K /var/db/pkg/net-ftp 280K /var/db/pkg/sci-libs 23M /var/db/pkg/dev-python 1.2M /var/db/pkg/app-alternatives 8.6M /var/db/pkg/sys-apps 1.8M /var/db/pkg/x11-apps 132K /var/db/pkg/tower-defence 284K /var/db/pkg/net-wireless 2.5M /var/db/pkg/app-arch 184K /var/db/pkg/games-arcade 208K /var/db/pkg/dev-debug 196K /var/db/pkg/net-firewall 4.0K /var/db/pkg/dev-ml 21M /var/db/pkg/dev-libs 1.4M /var/db/pkg/sys-boot 768K /var/db/pkg/www-client 17M /var/db/pkg/media-libs 1.1M /var/db/pkg/dev-lua 472K /var/db/pkg/net-fs 288K /var/db/pkg/perl-core 3.1M /var/db/pkg/media-gfx 228K /var/db/pkg/www-plugins 472K /var/db/pkg/kde-frameworks 292K /var/db/pkg/games-puzzle 4.5M /var/db/pkg/dev-util 216K /var/db/pkg/games-fps 3.7M /var/db/pkg/acct-group 960K /var/db/pkg/net-dns 152K /var/db/pkg/sci-mathematics 276K /var/db/pkg/x11-wm 860K /var/db/pkg/games-strategy 208K /var/db/pkg/app-emulation 752K /var/db/pkg/net-vpn 18M /var/db/pkg/dev-perl 320M /var/db/pkg