Apparently affects all Android devices and many different versions of the gmail app. I suspect it's server side. But it's interesting that there aren't any threads on XDA about it, at least from a discussion point of view. The only place online discussing this issue is this thread on the Gmail Support Forum:
tl;dr - major sync issue involving GMail only, affects native Gmail app by Google only, does not affect webmail or IMAP access to people's gmail accounts. not all Gmail users are affected, but the thread above is one of the busiest on the Gmail Support Forum at the moment.