Force SSL/TLS with 587 port is working for me. I have enabled TLS 1.2 and up only (no ssl on smartermail)

That used to be the case, but now it's the opposite: RFC 8314 recommends Implicit TLS (typically port 465) over STARTTLS (Explicit TLS, typically port 587).

Normal mail delivery occurs over port 25, so interoperability with Gmail is not an issue.   Client connections from Outlook or emClient use 465 or 587.  

The critical issue is your port configuration.   In Outlook, implicit TLS is labelled SSL/TLS.  In SM, it is labelled SSL (but with plans to match Outlook  labelling in a future release.)

Well it's all confusing due every company doesn't follow a standard. (Like RFC) If you google it it's more mess.

I use 993 TLS for IMAP and 587 TLS for SMTP (Force in both cases) and i'm super happy!

Typically port 465 was always associated with SSL, so I believe it's deprecation means that 587 (with STARTTLS) should be used in its place. Whether or not you disable SSL functionality will depend on your users and which protocols they support. Messages going into the junk email folder on third-party providers could be happening for a variety of reasons unfortunately - if you click on ...>Show Original on the received message in Google - do you see it failing any spam checks?