Okehampton Summer Sunday trains begin tomorrow (19 May) and with them will be a new bus service between Okehampton station and Gunnislake station connecting with some trains.
There is a maximum bus fare of £3 Single or Day Return for people with a valid rail ticket to Okehampton or Gunnislake (including Rovers, Rangers etc).
More information, including the bus timetable, here
http://www.dartmoorbus.com