The main transport services for the Galloway hills are the trains to Dumfries from Glasgow which pass through Sanquhar and buses to Dumfries and Newton Stewart

There are also bus services from Edinburgh and Glasgow and both pass through Moffat giving access to the Borders hills as well

Buses to Dumfries from Edinburgh here

Buses to Dumfries from Glasgow here

Newton Stewart is a base for the western hills in the area and may be reached from Dumfries by bus
Timetable here

From Newton Stewart there are buses to Glen Trool visitors centre
Timetable here

The bus does not go up to the car park for Merrick but hitchhiking is fairly easy

For Corserine, the Rhinns of Kells, and Cairnsmore of Carsphairn , there is a bus from Castle Douglas to Dalmellington
Timetable here

Castle Douglas is a stopping point of the Dumfries to Newton Stewart bus given above.
There is hostel accommodation in Parton on Loch Ken