Install this version:
emerge -a =media-sound/acousticbrainz-gui-9999
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =media-sound/acousticbrainz-gui-9999
Or alternatively:
emerge --autounmask-write -a =media-sound/acousticbrainz-gui-9999
# Copyright 1999-2014 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: $
EAPI=8
inherit cmake git-r3
DESCRIPTION="This client lets you submit your own audio features to the AcousticBrainz project."
HOMEPAGE="https://github.com/MTG/acousticbrainz-gui/"
SRC_URI=""
EGIT_REPO_URI="https://github.com/MTG/acousticbrainz-gui.git"
LICENSE="GPL-2"
SLOT="0"
KEYWORDS="~amd64 ~x86"
IUSE=""
DEPEND="dev-qt/qtgui:5"
RDEPEND="${DEPEND}"