The Schoolhouse at Ringford is a nice stop off for a lunch or coffee when you're on the A75, might be a bit too close to the GoF site to justify stopping as it's probably less than 10 miles to the campsite though.
Lovely place to stop for a snack is Kilnford Farm in Dumfries, if coming from the East it's the 6th roundabout on the Dumfries by-pass A75. They do really nice food and also sell lovely home bred butcher meat.
If anyone needs to stop for shopping then Aldi have just opened a new store right on the A75 at the large roundabout on the By pass, 4th roundabout coming east take the third exit and it's right there.