An other try:

dev-libs/gdbus-codegen/files/gdbus-codegen-2.56.1-sitedir.patch

@@ -0,0 +1,53 @@
+From 9eaaa76e2e36e46a43dbd419724696fd7ff8ea64 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?R=C3=A9mi=20Cardona?= <remi@gentoo.org>
+Date: Sat, 14 Apr 2018 09:55:22 +0200
+Subject: [PATCH 1/2] gdbus-codegen-2.54.3-sitedir.patch
+
+---
+ gio/gdbus-2.0/codegen/gdbus-codegen.in | 28 --------------------------
+ 1 file changed, 28 deletions(-)
+
+diff --git a/gio/gdbus-2.0/codegen/gdbus-codegen.in b/gio/gdbus-2.0/codegen/gdbus-codegen.in
+index 67d367543..190afa28f 100755
+--- a/gdbus-codegen.in
++++ b/gdbus-codegen.in
+@@ -20,36 +20,8 @@
+ # Author: David Zeuthen <davidz@redhat.com>
+ 
+ 
+-import os
+ import sys
+ 
+-srcdir = os.getenv('UNINSTALLED_GLIB_SRCDIR', None)
+-filedir = os.path.dirname(__file__)
+-
+-if srcdir is not None:
+-    path = os.path.join(srcdir, 'gio', 'gdbus-2.0')
+-elif os.path.basename(filedir) == 'bin':
+-    # Make the prefix containing gdbus-codegen 'relocatable' at runtime by
+-    # adding /some/prefix/bin/../share/glib-2.0 to the python path
+-    path = os.path.join(filedir, '..', 'share', 'glib-2.0')
+-else:
+-    # Assume that the modules we need are in the current directory and add the
+-    # parent directory to the python path.
+-    path = os.path.join(filedir, '..')
+-
+-# Canonicalize, then do further testing
+-path = os.path.abspath(path)
+-
+-# If the above path detection failed, use the hard-coded datadir. This can
+-# happen when, for instance, bindir and datadir are not in the same prefix or
+-# on Windows where we cannot make any guarantees about the directory structure.
+-#
+-# In these cases our installation cannot be relocatable, but at least we should
+-# be able to find the codegen module.
+-if not os.path.isfile(os.path.join(path, 'codegen', 'codegen_main.py')):
+-    path = os.path.join('@DATADIR@', 'glib-2.0')
+-
+-sys.path.insert(0, path)
+-from codegen import codegen_main
++from gdbus_codegen import codegen_main
+ 
+ sys.exit(codegen_main.codegen_main())
+-- 
+2.17.0

dev-libs/gdbus-codegen/files/setup.py-2.32.4

@@ -0,0 +1,6 @@
+from distutils.core import setup
+setup(name="gdbus_codegen",
+      version="@PV@",
+      packages=["gdbus_codegen"],
+      package_dir={"gdbus_codegen" : ""},
+      scripts=["gdbus-codegen"])

dev-libs/gdbus-codegen/gdbus-codegen-2.67.6.ebuild

@@ -0,0 +1,73 @@
+# Copyright 1999-2021 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=7
+GNOME_ORG_MODULE="glib"
+PYTHON_COMPAT=( python3_{7,8,9} )
+PYTHON_REQ_USE="xml"
+DISTUTILS_SINGLE_IMPL=1
+DISTUTILS_USE_SETUPTOOLS=no
+
+inherit gnome.org distutils-r1
+
+DESCRIPTION="GDBus code and documentation generator"
+HOMEPAGE="https://www.gtk.org/"
+
+LICENSE="LGPL-2+"
+SLOT="0"
+KEYWORDS="~alpha ~amd64 ~arm ~arm64 ~hppa ~ia64 ~m68k ~mips ~ppc ~ppc64 ~riscv ~s390 ~sparc ~x86 ~amd64-linux ~x86-linux ~x64-macos"
+IUSE=""
+
+RDEPEND="${PYTHON_DEPS}"
+DEPEND="${RDEPEND}"
+BDEPEND="
+	dev-libs/libxslt
+	app-text/docbook-xsl-stylesheets
+"
+
+S="${WORKDIR}/glib-${PV}/gio/gdbus-2.0/codegen"
+
+python_prepare_all() {
+	PATCHES=(
+		"${FILESDIR}/${PN}-2.56.1-sitedir.patch"
+	)
+	distutils-r1_python_prepare_all
+
+	local MAJOR_VERSION=$(ver_cut 1)
+	local MINOR_VERSION=$(ver_cut 2)
+	sed -e 's:@PYTHON@:python:' gdbus-codegen.in > gdbus-codegen || die
+	sed -e "s:@VERSION@:${PV}:" \
+		-e "s:@MAJOR_VERSION@:${MAJOR_VERSION}:" \
+		-e "s:@MINOR_VERSION@:${MINOR_VERSION}:" config.py.in > config.py || die
+	cp "${FILESDIR}/setup.py-2.32.4" setup.py || die "cp failed"
+	sed -e "s/@PV@/${PV}/" -i setup.py || die "sed setup.py failed"
+}
+
+do_xsltproc_command() {
+	# Taken from meson.build for manual manpage building - keep in sync (also copied to dev-util/glib-utils)
+	xsltproc \
+		--nonet \
+		--stringparam man.output.quietly 1 \
+		--stringparam funcsynopsis.style ansi \
+		--stringparam man.th.extra1.suppress 1 \
+		--stringparam man.authors.section.enabled 0 \
+		--stringparam man.copyright.section.enabled 0 \
+		-o "${2}" \
+		http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl \
+		"${1}" || die "manpage generation failed"
+}
+
+src_compile() {
+	distutils-r1_src_compile
+	do_xsltproc_command "${WORKDIR}/glib-${PV}/docs/reference/gio/gdbus-codegen.xml" "${WORKDIR}/glib-${PV}/docs/reference/gio/gdbus-codegen.1"
+}
+
+src_test() {
+	einfo "Skipping tests. This package is tested by dev-libs/glib"
+	einfo "when merged with FEATURES=test"
+}
+
+python_install_all() {
+	distutils-r1_python_install_all # no-op, but prevents QA warning
+	doman "${WORKDIR}/glib-${PV}/docs/reference/gio/gdbus-codegen.1"
+}