For email, there is a thunderbird addon called Enigmail which performs email encryption or signing, and is quite nicely integrated. Install it from the Software Center or with
sudo apt-get install enigmail.
Seahorse still exists (it is called "Passwords and Keys" if you search in the dash), but does appear to have lost built-in support for text encryption/signing (I think - I don't have a GPG key on this system to test with).
This thread gives the syntax for signing, encrypting, verifying and decrypting text in the console with the
This answer explains how to add gpg integration to
gedit, although I must admit I was expecting to find better plugin support for encrypt/sign in both
tomboy notes without obvious success.