On our Gentoo Packages site, we currently recommend using `gnutls`, however, `gnutls` is currently masked because of https://github.com/aria2/aria2/issues/1636.
Perhaps the Packages site info needs an update.
Where are you seeing gnutls as "recommended"?
This page simply lists the available USE flags; it makes no suggestions.
Looks like it's in metadata.xml:
~/g/net-misc/aria2 $ grep gnutls metadata.xml
<flag name="gnutls">Use <pkg>net-libs/gnutls</pkg> as TLS provider (preferred). Otherwise, <pkg>dev-libs/openssl</pkg> will be used as TLS provider. Both apply only with USE=ssl.</flag>