Gmail’s offline access feature gives you your recent messages while you’re on a plane or otherwise unconnected. Now you can get speedier offline access, or deeper archives, by customising just how long to store messages.
Offline Gmail’s settings default to 6 month access for your inbox, with a few variations for starred messages and less important folders, like spam. Now you can get a bit more specific, scaling down offline storage ranges for speedier offline access and syncing, or adding in more messages for a bigger pool to draw from. Gmail makes the trade-off between storage and speed explicit if you try to push so many mail labels to storing “all”:
Having more control over your offline access is a good thing all around, though, and lets those who are just jumping offline for short periods to minimize the drag on their Gmail page loading.
Choose which messages get downloaded for offline use [Official Gmail Blog]