Firstly, Keybreeze is excellent. Secondly, I wasn't aware that plugins were available - some look great, but they need to be advertised more. 
However, I was a bit miffed to find - AFTER installing the gcal plugin - not only do I need to download and install cURL and Twitter (ok it does say "... uses Twittercal, cURL, and Google's Quick Add feature..." but I must allow Twitter to access to my Google Calendar. Presumably, that means Twitter has to be running all the time? I think I'll stick to the Googlecal plugin for Firefox - install, place the icon, and click when needed. Thanks anyway.