stunnel ebuild does not provide .service file. There is a template in tools/ folder.
+*stunnel-4.56-r2 (22 Dec 2013) + + 22 Dec 2013; Pacho Ramos <pacho@gentoo.org> +files/stunnel.tmpfiles.conf, + +stunnel-4.56-r2.ebuild: + Add unit file (#493978) +