The island of Gavdos is 45 kilometers south of Chora Sfakion and can be reached by a small ferry boat (with a very, very noisy engine) from Chora Sfakion, or from Paleochora. Bring earplugs :), haha!! Update: in 2011 a new faster and more quiet boat will be going up and down to Gavdos.
If you want to know the time table for the ferries from Paleochora, Sougia or chora Sfakion to the island of Gavdos (and back) you can check the website of Selino Travel (Tel.: + 30 28230 42272 - email). Usually most of the ferry traffic from Chora Sfakion takes place around the weekends and from Paleochora during the weekdays, but this could of course change.
For the ferries to Gavdos and the timetables check the website of Anendyk lines.