All ebuilds must specify a LICENSE (note the American English spelling) variable. The license names listed in this variable must match files existing in the repository's licenses/ directory.
LICENSE="LGPL-2.1+ tools? ( GPL-2+ )"
If the package sources include additional files that are not installed, their license should not be listed. However, if those files are used at build time, then the license must not impose any restrictions that could prevent users from building the software. Please also note that some licenses may impose additional restrictons, e.g. fetch and/or mirroring restriction.
LICENSE="|| ( foo bar )"
Warning: Do not trust the license indicated by GitHub or other sites that use automation to guess the license! The information presented may be incomplete and/or incorrect.
To establish the correct value of LICENSE, you need to trace the licenses of all installed files. Normally, the licenses of output files (compiled executables, generated files) are implied by the licenses of the relevant input files.
The latter (more specific) options take precedence over the former. In particular, COPYING* files are frequently included as hardcopies of applicable licenses but the exact application of licenses and their versions are specified elsewhere.
Please watch for license conflicts. If the license indicated by the package is incompatible with the licenses used by its sources (e.g. BSD/MIT package including GPL sources), please contact the licenses team for guidance. Do not add packages that seem to include license term violations.
FSF licenses (GPL, LGPL, AGPL, FDL) occur in two variants: the 'vN only' and 'vN or later' variants. In Gentoo, the licenses of the latter variants are denoted by appending a plus sign (+) to their respective license notations of the former variant, e.g. GPL-2+ and GPL-2.
If your package's license is not already in the tree, you must add the license before committing the package. When adding the license, use a plain text file (UTF-8 encoded) if at all possible. Some licenses are PDF files rather than plain text — this should only be done if we are legally required to do so. Finally you need to check if your license should be added to a license group as listed in $PORTDIR/profiles/license_groups. For more information see the GLEP 23.
It is not normally necessary to mail the gentoo-dev list or licenses@gentoo.org before adding a new license. You should only do so if the license could be considered 'questionable' or if you are unsure as to the meaning of any part of it.