As a Gentoo user, as my regular desktop, I have to say that both the answers here, are only half correct.
[I] sys-libs/glibc Available versions: (2.2) 2.13-r4^s 2.14.1-r3^s 2.15-r3^s 2.16.0^s 2.17^s ~2.18-r1^s ~2.19^s 2.19-r1^s ~2.20^s ~2.20-r1^s 2.20-r2^s **2.21^s **9999^s {debug gd hardened multilib nscd profile selinux suid systemtap vanilla CROSSCOMPILE_OPTS="headers-only"} Installed versions: 2.20-r2(2.2)^s(10:40:38 PM 04/04/2015)(gd multilib -debug -hardened -nscd -profile -selinux -suid -systemtap -vanilla CROSSCOMPILE_OPTS="-headers-only") Homepage: http://www.gnu.org/software/libc/libc.html Description: GNU libc6 (also called glibc2) C library
If you read the Installed Version, you'll see that version 2.20-r2 of the glibc system library is installed which fills slot 2.2. This is confirmed by the output of cat /etc/gentoo-release which outputs:
Gentoo Base System release 2.2
as you've noted.
Explanation
The other answers are correct in that Gentoo is a rolling release. The release number is chosen by the version of sys-libs/glibc because every compiled package on the system depends on glibc. As such, this is how Gentoo chooses the release number. This is also true for every binary distribution like Ubuntu. All packages chosen by the maintainers, all depend on a chosen version of glibc. If you look at the Glibc GNU Software Page, you'll read:
Overview
Any Unix-like operating system needs a C library: the library which defines the "system calls" and other basic facilities such as open, malloc, printf, exit...
The GNU C Library is used as the C library in the GNU systems and most systems with the Linux kernel.
Project Goals
The GNU C Library is primarily designed to be a portable and high performance C library. It follows all relevant standards including ISO C11 and POSIX.1-2008. It is also internationalized and has one of the most complete internationalization interfaces known.
Knowing this, and seeing that Version 2.20 was released on 2014-Sept-7, any release you download from the Gentoo Downloads, that's dated past 2014-Sept-7 will result in the proper release to start with. After downloading this, you must follow the proper Gentoo Handbook for your architecture. You can use the Hybrid CD, which the other posters referred to as the Live DVD Release, or the minimal CD, which your ISP used to create their own server with the handbook. As other posters have noted, with a rolling release this version number may change as you update the system using portage.