As of 7/22/15, 1:17 PM, Google reports the issue is resolved.
Over on the mailop list multiple people are reporting delivery problems to Gmail.
The Google status page confirms this:
7/22/15, 12:14 PM: We’re investigating reports of an issue with Gmail. We will provide more information shortly. Users may experience message delivery delays.
These delays are happening over both IPv4 and IPv6. Some attempts to send mail to Gmail MTAs are receiving the message:
Deferred: 421 4.7.0 Temporary System Problem. Try again later (MU).
I expect this will resolve soon, but right now, expect slow sending to Gmail and Google Apps hosted domains.