One of the longest-running requests for Google’s web mail service Gmail has been for offline functionality. Now, finally, Gmail users will be able to type up those emails inside an airplane. Google has just announced offline Gmail support via Gmail Labs – to start with for consumers and businesses using Google Apps, but regular Gmail consumers will get it over the next couple of days. The offline feature was built using Gears, Google’s offline web application API.

In its email to us, Google archly noted that Internet connectivity has started to happen even in airplanes – for example American Airlines announced last year that they will offer in-flight wi-fi service. Even so, many people (including this author) have been clamoring for offline Gmail for years. Google’s web mail competitors have been much swifter in offering offline functionality. In October 2008 we reported that Zoho Mail had gotten offline support, ironically via Gears. Yahoo Mail gave offline access to all free and paid Yahoo Mail users through the Yahoo Zimbra Desktop in July ’08.

It’s been frustrating for Gmail users, many of whom are early adopters, that Gmail has been so behind the times with this feature. Particularly when Google has Gears and has already used it to offline-enable Google Docs and Google Reader. So Google has been slow to use Gears in Gmail and Google Apps, but of course we’re pleased to see it being deployed now.

With its new offline access, Gmail will be able to be used inside the browser even without an Internet connection. Messages can be read, starred, labeled and archived and new mail can be composed, says Google. Messages ready to be sent will wait in the Outbox until the user is online again when the messages will be sent automatically. Google has warned though that “there might be some issues that aren’t completely ironed out” in the new feature.

The feature will be rolled out over the next couple of days, starting this afternoon for Google Apps users. You can enable offline Gmail by clicking ‘Settings’ in your Gmail, then click the ‘Labs’ tab and select ‘Enable’ next to ‘Offline Gmail’, and click ‘Save Changes.’ Then in the upper righthand corner of the account, next to the username, there will be a new ‘Offline’ link. Click this link to start the offline synchronization process (note that you will need to to download Gears if you haven’t already). For non-Google Apps users, you’ll have to wait a couple more days.

But wait, there’s more. Google is also working on an offline Google Calendar. There is no date for this release, but it too will be launched with Google Apps first.