I do understand Google's challenge with handling probably the largest amount of email traffic in the world, and then the most spam in the world, and that it is a continuously moving target.
But it does feel like they are treating nearly all minor relays as spam relays. We can not block 100% of spam before we relay onwards to Gmail as that would mean too many false positives emails get blocked, but most try to block as much as possible.
In my case, my servers probably block 99% of spam, but some will get relayed, and most of those ends up being handled by Gmail's even better Bayes scoring and filtered to end users' spam folders.
I don't know what the automatic threshold limit is to be on their naughty list, but it must be very low, as in double digits per day.
I no longer use Gmail myself but half of my users relay some aliases to their main Gmail account. No problems with that, except my servers continuously get rate limited by Google:
I have the normal anti-spam features set up (SPF, DKIM, DMARC, Postgrey, Spamassassin, etc) so most spam gets rejected but not all. But several of my users have very common English names as their aliases that are already guessed and added to many spam databases.This leads to family phoning me all the time to say my server is broken when it is just Google throttling everyone's emails. I frequently have to check Postfix queues and clear some obvious spam or just pointless Facebook update emails which seems to be the majority... (Yes your email admin can read your email...)
The only way to improve is to constantly remind my family to not sign up to all crap, use not so common aliases, and try to keep tightening my anti-spam configurations. But we really are not talking about a lot of emails. Probably less than a hundred per day spread across 5-ish end accounts of which 95% is probably legit. Yet Google is treating me as some totally open relay. (╯°□°)╯︵ ┻━┻