Gmail Offline usually stores its files under your profile path.
You can find your profile path by typing chrome://version in the address bar, and searching for Profile Path.
The path is usually:
/Users/%username%/Library/Application Support/Google/Chrome/Default/
Alternatively, if you are logged in under the account you want to explore, the path is:
~/Library/Application Support/Google/Chrome/Default/
If you have a Chrome user profile other than the default one, replace Default with that profile name.
You should then look at the "Local Storage" and "Local Extension Settings" directories under that path.