It's all you really need...
As of this week, Gmail has reached perfection: You no longer have to be online to read or write messages. To get offline access, you first need to download and install a small program called Google Gears (except if you're using Google's Chrome browser, which comes with Gears built in).*
Then, after you enable Gmail's offline capability, the system will download two months of your most recent messages, which should take 30 minutes to an hour.
Now you're good to go.