The US firm confirmed that it had suffered an outage leaving users, including people and companies in the UK, unable to send or receive messages.
It remained unclear what triggered the outage, which also affected users in the US and India, and prompted a stream of complaints on social media forums.
Google was initially unable to provide any information before suggesting the service was back.
Two other Google sites, the social network Google Plus, and YouTube, appeared to load slowly as well.
The news led to an outpouring of comments on Twitter, with some users comparing the service suspension to an apocalypse.
Rival Yahoo posted Gmail error page on Twitter pointing out the mistake message and Google's note: "We're sorry, but your Gmail account is temporarily unavailable. We apologise for the inconvenience and suggest trying again in a few minutes."
Google later added on its website: "The problem with Gmail should be resolved.
"We apologise for the inconvenience and thank you for your patience and continued support.
"Please rest assured that system reliability is a top priority at Google, and we are making continuous improvements to make our systems better."