It’s dead simple to access your Gmail even if you’re not connected to the Internet. All you need to do is download and install the Gmail Offline app and set your Gmail account to support cached emails (as well as a cached version of your address book and recent attachments). Here’s a step-by-step on how to set up Offline Gmail for your Chromebook.
If you want to compose a message, select the Pencil icon above the Inbox.
Your address book is cached, so a list of recipients will autocomplete in the To:, CC: and BCC: fields, similar to using Gmail online.
Hit Send after you compose your message.
When you connect to the Internet again, the message will send automatically. You can confirm in your Sent folder.