Gmail usually stores its data on internal storage:
│ ├── download
│ └── webviewcache
From this directory structure you may guess some things. I'm not sure what exactly is stored in the
cache subdirectory except for downloads (the extracted Nandroid backup I've got on my workstation comes from a device where Gmail actually wasn't used). Your messages (in fact, your entire mail contents) are however stored in a SQLite3 database:
databases/mailstore.<address> -- where
<address> is your mail address.
So if you can pull this directory to your computer (which might require root privileges), you could use a graphical SQLite frontend to explore this database. The
messages table will be your primary target.