Blimey - this was way harder than it should have been...
OK - I just want to be able to quickly encrypt/decrypt passwords - and to be able to encrypt the password using other people's public GPG keys - should be easy!
First - make sure the keys have been imported OK. First problem - if you use the standard Seahorse key manager the keys may be imported - but they won't show. Use the command line with gpg2 to import the keys - for example:
$ gpg2 --import username_gpg.asc
Import all the keys you want to encrypt/decrypt with. Ususally this is your own private/public key pair - and the public GPG keys of other users. Importantly, make sure you can see the keys in Seahorse.
The problem with Seahorse importing keys is logged at https://bugs.launchpad.net/ubuntu/+source/seahorse/+bug/1577198
Now install Geany and its GPG plugin - honestly - it's the best way - gedit won't work!
# apt-get install geany geany-plugin-pg
Geany is very lightweight - so it doesn't feel like its bloating your system up.
Go to Tools - Plugin Manager and enable the GeanyPG plugin.
Then - use an editor to decrypt/encrypt. The options are under Tools - GeanyPG.