Theres a forest literally 2 miles north of Gatehouse, once your into it, head north west and theres an old railway line, it heads off in the direction of Moffat. Huge viaduct up there, and loads of old damns and interesting stuff.
Riding isn’t amaze balls, mostly just very quiet overgrown forest track, but its quite scenic and the viaduct is fun to play on.
Also theres a country house estate within Gatehouse (between the town and straddling the main road) which has waymarked trails in it, its always quiet and once your under the main road (between road and the beach) you generally have the place to yourself. I think there was an area of bombholes which make for hours of jumping chaos if the mood takes you.