Currently you can only find out that Portage uses GPL-2 by looking at individual files. Let's have a LICENSE file too.
Thanks, this is in git now: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=0b5d13e1d8510264b6f61bb9c8cb8d80c18bfcb2
This is in 2.2_rc68, but I'll leave this bug open until it's in an unmasked version.
This is fixed in 2.1.9.