Hello!
In Visby, where you will arrive with the ferry, there are some bike rentals, close to the harbor. If you got a tent, there are lots of good places to camp, alone in the "bush" or at big camps. Buy a good map, collect some information in your language (from internet) and keep close to the sea. You vill love it.
Its possible to catch a bus just to get a little bit further. I prefer the north of Gotland!