Google says the issue is affecting a "significant subset" of users.
Most users can access their inboxes but may encounter "error messages, high latency, and/or other unexpected behavior," according to an update released by the company on its service status page.
The email issues come just a day after Google also experienced an outage to its other services, including YouTube and Google Docs.
In an update, Google says "Gmail service has already been restored for some users". "We expect a resolution for all users in the near future. Please note this time frame is an estimate and may change. The affected users are able to access Gmail, but are seeing error messages, high latency, and/or other unexpected behaviour," the company added.