I've downloaded the Zend GData package to use the Google Calendar API. When I look through the contents of the package it seems to contain loads and loads of stuff. Do I really need all of it just for using Google Calendar and no other Google APIs? If not, what can i safely get rid of?
You probably won't use many of the files in Gdata folder, but Zend_Gdata_Calendar
extends the Gdata class, and Gdata extends Gdata_App which uses Zend_Http_Client (and a few other classes) to (essentially) make requests to google. Loader loads classes, Registry stores objects/data, and so on. So, yeah, many of the downloaded files will be used. You could of course rewrite much of the code to use only the parts you need, but that kind of defeats the point of using the framework.
I'd recommend building a prototype of your application following the tutorials on Zend but with all the library in place. Once you can see how it works, you can more easily rip what you don't need. At least it's just the Gdata library and not the whole 64MB full framework :D