Here we go: a very, VERY brittle ebuild: never tested, never built. Just copied-and-pasted from gtk-sharp-2.12.45.

Good luck! *duckandcover*

dev-dotnet/gtk-sharp/Manifest

@@ -1,4 +1,6 @@
 DIST gtk-sharp-2.12.45.tar.gz 2644209 BLAKE2B 1455e1e5594b1b91851621b37a15932cd498857c19dab51c8434f764b3918f7c5ca47674eb5089ebecfc8e55850151a642617623744cdbce1566b158af998e54 SHA512 6e11a6a3e8f7f3f9511c674cdcbda15274e597b8e72bc2cdd6970f8906e295849a94108794c11b9aef5408631bf10d2dd7d855d5e5081f58fc790e7194f6159f
+DIST gtk-sharp-2.99.3.tar.xz 1524220 BLAKE2B cdfd3f40ad3623e5a542ca36a203ad576e4d8eea6b37f79cc89ccfa92dbb853034a9946afb2494057a714547c54a465863c3ec14b7ab7c921bb65c1d5a03ce47 SHA512 31c132b411f4602cdc71e0dfe94bd1f2884f62f2d33e9aeae9eb9c3cd2a58c2d503b06d4b5ac08b3e396b89810669360305c4bc040bce57d22e2eed48c07fb0c
 EBUILD gtk-sharp-2.12.45.ebuild 1383 BLAKE2B 5e267a93b420f62d103c7d79dd16420defcfc4eda26e0ba9ee8d67730759db1c1536483cbfab99a3c5b66597e8694ce18373a6fd182e329a6e4016589ec4632b SHA512 8e105219a91781ca4b6d3256e38f57a26be4abc8fcab8b7537f89a39cf333e728917c86bea4ac7c5e1ac3106e9336c81629d62b595718d2608401fafd83c1ebe
+EBUILD gtk-sharp-2.99.3.ebuild 1157 BLAKE2B 5b5d7043e2164ff22ae9249e455b06f8619b3dfe7b168f3f594e0763ca92954a6fbd7c2c03efaf20e1a41bba0764ca1593355ac6442b8f9f68e9e26fb5f38009 SHA512 aa97f92c5d28b235af5cdb25d765e6aaa29175bab29e896813018a4ea70aed4195e9829e792bffa44e7c4f1f4e816e8fa6256c98f6857acf40090747358712e1
 MISC ChangeLog 184 BLAKE2B 82098e9eb5b24207855e9b901e7467e960ff6438ca5c407d3b5260015b2624b0c143bc5f1a944e7d8a883003e8e14f9827306a7fb2916904972b09ad7b570e53 SHA512 8b78609d5375b4dd340c49db4a68c2df599b803e391a118a97009f258176d716cbced02fbc30fa2f6bae1a0ec3ed54151cc58eab03af3659a8aba18cd2a28336
 MISC metadata.xml 248 BLAKE2B cb50cad2c614a99daa2e236f9e0d0fab1ab20a898c0b0f0c28a1387820ab1f2b9dd4d396a3b38f732b493824f96a29ee5894e80e09dbd08dff62c1aaba3feaa0 SHA512 29f9b96b0a77546cce6a8bffd82fae2fd0939d32a7a05ffd0d91f9250a019a3ddaa599b93988c82daabb9c4c4b284dd947e2c47f6ca7727b8539cffa91bcb8c2

dev-dotnet/gtk-sharp/gtk-sharp-2.99.3.ebuild

@@ -0,0 +1,58 @@
+# Copyright 1999-2020 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=7
+
+#inherit dotnet autotools base
+inherit dotnet autotools
+
+SLOT="3"
+DESCRIPTION="gtk bindings for mono"
+LICENSE="GPL-2"
+HOMEPAGE="http://www.mono-project.com/GtkSharp"
+KEYWORDS="~amd64 ~arm64 ~ppc ~x86"
+SRC_URI="https://ftp.gnome.org/pub/GNOME/sources/gtk-sharp/2.99/${P}.tar.xz"
+IUSE="debug"
+
+RESTRICT="test"
+
+RDEPEND="
+	>=dev-lang/mono-6.6
+	x11-libs/pango
+	>=dev-libs/glib-2.31
+	dev-libs/atk
+	x11-libs/gtk+:3
+	dev-perl/XML-LibXML"
+DEPEND="${RDEPEND}
+	virtual/pkgconfig
+	sys-devel/automake:1.11"
+
+src_unpack() {
+	mkdir "${T}"/the_registry
+	default
+}
+
+src_prepare() {
+	eautoreconf
+	_elibtoolize
+	eapply_user
+}
+
+src_configure() {
+	econf	--disable-static \
+		--disable-maintainer-mode \
+		$(use_enable debug)
+}
+
+src_compile() {
+	MONO_REGISTRY_PATH="${T}/the_registry" emake
+}
+
+src_install() {
+	default
+	dotnet_multilib_comply
+	sed -i "s/\\r//g" "${D}"/usr/bin/* || die "sed failed"
+	dodir /etc/mono/registry || die "failed to create registry dir"
+	insinto /etc/mono/registry
+	doins -r "${T}"/the_registry/* || die "failed to install to registry"
+}