Check with your airline carrier for their recommendations and package weight and size limits, and with shipping companies servicing both France and Guadeloupe.
I'd generally want to pack the iMac in the original box, or have the shipper pack and insure it. After Plus is listed as an Apple reseller in Guadeloupe.
Depending on the vintage of the iMac, selling your current iMac and purchasing one in Guadeloupe might be cheaper than shipping, though I'd check with the Apple reseller to determine models and availability before trying that.
Given what I've seen of airport baggage handling practices around the world, shipping a 27" iMac by airline passenger cargo hold would not be my first choice. Particularly if the original carton or equivalent packaging isn't available. I've seen ground crews throw luggage from the hold onto the ground among other misadventures, and folks traveling with me had cargo stolen between Paris and Lyon, among other legs.
Also confirm that the proper power cord for Guadeloupe is available, though it does appear that both France and Guadeloupe use the same electrical connections.