Install this version:
emerge -a =sci-libs/sgp4-20210111-r1
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =sci-libs/sgp4-20210111-r1
Or alternatively:
emerge --autounmask-write -a =sci-libs/sgp4-20210111-r1
# Copyright 2021 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
inherit cmake
COMMIT="ca9d4d97af4ee62461de6f13e0c85d1dc6000040"
DESCRIPTION="Simplified perturbations models"
HOMEPAGE="https://github.com/dnwrnr/sgp4"
SRC_URI="https://github.com/dnwrnr/sgp4/archive/${COMMIT}.tar.gz -> ${P}.tar.gz"
S="${WORKDIR}/${PN}-${COMMIT}"
LICENSE="Apache-2.0"
SLOT="0"
KEYWORDS="amd64 x86"
PATCHES=(
"${FILESDIR}/${PN}-libdir.patch"
)