The Great Glen Way

Well, the website says this...

Currently, cyclists use the original Great Glen Way itself with great success. However, the surface is not really suitable for road tyres. We have cyclists regularly completing the route on mountain bike or robust hybrid. It is fairly direct, and south of Fort Augustus there are few hills

and judging by what I saw in this programme you could probably get away with just fat touring tyres on a road bike. The bigger the better I reckon, cos you'll avoid pinch flats