From what I can tell, in all my searching and tests, if I set up my own jabber server (using ejabberd) I can not do s2s with TLS to and from google XMPP servers.
When I connect, the ejabberd log records:
I(<0.377.0>:ejabberd_s2s_out:365) : Connection established: XXXX.net -> gmail.com with TLS=false
Is it even possible to do S2S TLS with google XMPP servers?
No, Google does not support TLS on server-to-server connections today, it was turned off some years ago. See for example this mailing list post.
It used to work, but there has been no official word on why it was disabled, or whether it might return.