The friendly GS bunch in Scotland helped me with a similar trip a couple of weeks ago - their advice was
Starting at BMW Bishopbriggs in Glasgow head to Lennoxtown via Torrance. Then up into the Campies to Fintry. (cracking B road with good surfaces). At Fintry take the B822 towards Kippen. A few miles along take a left turn signposted to Amprior. At Amprior turn right onto the A811 then first left onto the B8034 to Port of Monteith. Turn left at Port of Moneith onto the A81 and head to Aberfoyle taking the A821 over the Dukes Pass which is fantastic of a GS. Halfway along you have the option of following the 3 lochs forest trail which is well suited to a GS. The road after Brig o Turk has some nice humps and I got my GS airborne quite a few times on it yesterday
At Kilmahog you turn left onto the A84 and head North to Lix Toll where you take a right towards Killin. Some lovey waterfalls and rapids here so worth a stop for some photos. Follow the A827 for a few miles and take the single track road over by Ben Lawers signposted to the Bridge of Balgie. Stop there and have a Bowl of Soup at Boxertools Bench.