Run SpamAssassin on your Gmail inbox. Move spam to Gmail spam folder.
Install imapfilter & spamassassin.
Set up an app password on your Gmail account - https://support.google.com/mail/answer/185833?hl=en-GB
Create ~/.private/imapfilter/gmailcreds.lua like so:
gmaccount = IMAP {
server = 'imap.gmail.com',
username = '[email protected]',
password = 'yourapppassword',
ssl = 'tls13'
}
If you don't want to run the spamassassin daemon (spamd), change spamc to spamassassin in gmailass.lua.
Run imapfilter -c gmailass.lua. Maybe schedule it in cron.