BackUp Gmail gives you the opportunity to browse your emails offline in a simple way: search them by date and view by Quick Look.
Open the “Backup folder” (selected in Preferences) in the Finder. You can find the “FileStore” subfolder, where we stored all the emails. Browsing it you can see the folders structured by Year/Month. So you can browse month by month your emails and for each of them you can open a preview by Quick Look (select an email and press the Space bar).
Another way to look for your emails is to use the Spotlight. Even there you will see a preview (body and attachments) as you can see in the follow image: