I use some publicly available spam IP blocklists and my server started rejecting mail from gmail.com because they ended up on one of the lists for sending spam. I thought it was funny.

When this happens, gmail informs the sender that the mail wasn’t delivered and they try a few more times before telling the user that no more attempts will be made.

> When this happens, gmail informs the sender that the mail wasn’t delivered and they try a few more times before telling the user that no more attempts will be made.

That depends entirely on how the receiving server rejects the mail. If it just drops it into the spam folder or /dev/null Gmail won't be able to know. If it responds with a non-transient error code then Gmail will (correctly) NOT try again and inform the user of that.

Is it from GMail servers, or is it from spoofed GMail accounts?

From mail.google.com servers, dkim signed and all.

No, it's actually truly sent using GMail. Otherwise, it could not benefit from GMail's reputation.