The circlip has a notch at the end to allow you to prise it out with a small screwdriver.
I have a different model, but on mine, there is no circlip. If you look down the hole on the top, you will see a screw that needs to be undone.
That's what I have, a screw down the hole but I unscrewed it and couldn't lift the body off so assumed this related to something else?
See the existing thread: http://www.ybw.com/forums/showthread.php?498970-Servicing-a-Gibb-winch-Is-it-worth-it and particularly Post 5.
JD. So sorry. Didn't realise you had provided such a detailed response to the original post. Thanks a lot for this.