The Gentoo Reference System (GRS) Suite is a set of tools for building and maintaining a well defined Gentoo system in which all choices in building the system are predefined in configuration files housed on a central git repository. All systems built according to a particular GRS spec should be identical. As a "from source" distribution, Gentoo allows a large degree of customization. The space of all possible packages and USE flags is vast, not to speak of more radical choices such as different kernels (eg. Linux or FreeBSD), executable formats (eg. ELF or Mach-O), different C Standard Libraries (eg. glibc, uClibc or musl) and different providers of core userland utilities (eg. busybox or coreutils). In contrast to other distributions where each instance of an installed system is nearly identical to another, the large array of choice in Gentoo means that any two systems are unlikely to be sufficiently similar that executables or libraries from one will "just work" on the other, even if the architecture and other hardware factors are the same; assuming, of course, there is no conscious effort to build identical systems. This is where the Gentoo Release System (GRS) Suite comes in. It does precisely this, namely, it provides an automated method for repeatedly and predictably generating identical Gentoo systems.
GRS is designed to work roughly as follows: Periodic release tarballs are generated which are "polished". For example, releases can provide pre-configured display managers, windowing systems and desktop themes, even user accounts and home directories. Installation should be as simple as unpacking the release tarball on pre-formated partitions with minimal or no further configuration. There is no need to build any packages or a kernel and everything is ready to go out of the box. A release tarball can be downloaded from some server or alternatively can be built locally. While these may not initially be identical because they were build at different times, after updating, both approaches should yield identical systems.
Updating a GRS system can proceed by either building packages locally, or downloading pre-built binpkgs. As long as one does not deviate from the GRS defined set of USE flags, maskings and other configuration parameters, both approaches should yield identical systems. A GRS system is always a Gentoo system, so at any time, one can elect to part ways from GRS and venture out on one's own! The GRS Suite provides a utilities to make sure that configurations in /etc/portage are properly maintained in a manner consistent with GRS, but emerge and other portage utilities will always work. Even if one does deviate from the GRS specs, it should be possible to return to strict GRS using the Suite's utilities, provided one has not deviated too far.
GRS is provided by the app-portage/grs package. The same package is installed on either a server responsible for building the release tarballs and binpkgs, or on an actual GRS system. On the server, a daemon called grsrun will either do a release run, in which case it builds the release tarballs, or it will do an update run where it either builds or updates a bunch of extra binpkgs. For example, for GRS = desktop-amd64-uclibc-hardened, the release run builds a little under 900 packages and produces the polished release tarball, while the update run builds/updates about 5700 packages. The first update run after a new release is time consuming because some 5700 new packages must be built, but subsequent update runs need only build packages which were bumped since the last update run.
On the client, a utility called grsup acts as a wrapper to emerge. grsup will both manage the configuration files in /etc/portage as well as either builds or download the binpkg from a PORTAGE_BINHOST. After the initial installation of a GRS system, one need only run grsup to bring it up to date. While releases tarballs will be pushed out periodically, these are not used to update an existing GRS system, only to start new one. Rather, one GRS release should smoothly update to the next; in other words, each GRS release is equivalent to the previous release plus any updates since. The only reason to push out a new release tarball is to avoid having to subsequently push out a large number of updates for each new installation.