Here is a ebuild to install the SoapyAirspy package in gentoo, perhaps from a local overlay.
If you're familiar with gentoo, then you too should be able to easily synthesize this ebuild from the existing net-wireless/soapy[mumble] packages, which is all that I've done here.
$ cat /usr/local/portage/net-wireless/soapyairspy/soapyairspy-9999.ebuild
# Copyright 2020 <jsled@asynchronous.org>
EAPI=6
inherit cmake-utils
DESCRIPTION="SoapySDR Airspy Support Module"
HOMEPAGE="https://github.com/pothosware/SoapyAirspy"
KEYWORDS="~amd64"
if [ "${PV}" = "9999" ]; then
EGIT_REPO_URI="https://github.com/pothosware/SoapyAirspy.git"
inherit git-r3
else
# KEYWORDS="~amd64 ~x86"
# SRC_URI="https://github.com/pothosware/SoapyRTLSDR/archive/soapy-rtlsdr-${PV}.tar.gz -> ${P}.tar.gz"
# S="${WORKDIR}"/SoapyRTLSDR-soapy-rtlsdr-"${PV}"
echo "only 9999 supported"
fi
LICENSE="MIT"
SLOT="0"
RDEPEND="net-wireless/soapysdr:=
net-wireless/airspy"
DEPEND="${RDEPEND}"