It's nice to find a site that you don't have to walk 3 miles over moorland to visit.
The Galloway Stone is just such a site. Simply leave the M6 at Shap, turn L for Orton, then take the R turn to Salterwath just before the M6 underpass.
The Galloway Stone lies c. 50 yards to the L after 150 yards along this lane.
It looks like a large chunk of Shap Pink Granite so it can't be an erratic from Galloway.
The explanation for it's name may be that it was a marker on the Old Scotch Drove Road for turning onto the Galwaithegate or Galloway Road.
As an additional point of interest there is an Ordnance Survey Bench Mark Bolt on The Galloway Stone denoting a height of 98.8m.