Install this version:
emerge -a =app-emacs/alert-9999
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =app-emacs/alert-9999
Or alternatively:
emerge --autounmask-write -a =app-emacs/alert-9999
# Copyright 1999-2021 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
NEED_EMACS="22"
inherit elisp git-r3
DESCRIPTION="Growl-style notification system for Emacs"
HOMEPAGE="https://github.com/jwiegley/alert"
EGIT_REPO_URI="https://github.com/jwiegley/alert.git"
EGIT_BRANCH="master"
KEYWORDS="~amd64 ~x86"
LICENSE="GPL-3"
SLOT="0"
RDEPEND="
>=app-emacs/gntp-0.1
>=app-emacs/log4e-0.3.0
"
SITEFILE="50${PN}-gentoo.el"