Recently released, Gmail labs feature lets you check you mails in offline mode or without internet connection making email clients like Microsoft Outlook and Thunderbird obsolete to some extent. As Gmail, infact Google, has a practice of launching all its new products and experimental services under ‘labs’ tags, this time too Gmail has followed this way.
To start using this new lab feature, you need to enable ‘Offline Gmail’ option in Gmail Settings-> Labs tab. You will then need to install gears (if you don’t have it) and then you can start using Offline Gmail option. This feature is yet to be rolled out for every Gmail user (even mine) so don’t worry, soon the feature will be rolled on your profile too. 
As this feature is yet to be rolled on to my account too, so I cannot say you much about it. I will try reviewing this good feature once I practically try it out. Till then keep viewing this official video by Gmail.
Link: Official Release | Google Gears
As of now, it’s available only for US and UK residents.
@Dnyanesh
When I checked last night, this feature was not there in my gmail account.
But few hours back, I could see at and also using it now!
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