Spam assassin is enabled by default and I too noticed how decent gmails spam collection is in comparison to spam assassin. I did move some mails over to junk and it seems like leaving them there did somehow train spam assassin but it does take awhile.
Gmails spam filtering was just way better.
Also, If you haven’t already I do recommend looking at —>
This will ensure mail you want to receive quickly doesn’t get caught up with postgrey.