Mozilla Thunderbird with IMAP synchronisation with Gmail is a feasible alternative to Gmail web interface. Staying online within Google office with a one-click access to documents, RSS subscriptions and photos is nice. However there is one case in which the web interface fails: if you want to seriously use encryption.
In such cases GnuPG + Mozilla Thunderbird + Enigmail work fine. However, you immediately face a problem: if you’re normally using online interface, your contacts are in Google Contacts, and Thunderbird doesn’t know them.
To get access to your addressees in the Google Contacts, use Google Contacts add-on for Thunderbird.
It supports groups of contacts, postal addresses, telephone numbers although in a bit less flexible manner than Google Contacts does.