Lowering that mast is not for the faint of heart. It is a 3 man job. Once it starts down you need to control it I performed that operation each winter on our former GB 36 and it took 3 of us to do it without hurting ourselves.
There are a number of ingenious ways to do it more easily. Most involve a block and tackle arrangement going forward from the mast to an anchor point near the helm station on the fly bridge. Every GB that I know of came with a crutch to support the lowered mast. If you bought the boat used, check with the PO and search the boat lazarette and engine room spaces thoroughly.
You will find descriptions of the tackle arrangement on the GB site IAGBO. I have had no difficulty logging on to that site. Howard
Magic, 1996 Grand Banks Europa
Westport, CT and Stuart, FL