Interesting thread as like Trekster I have been exploring some of the areas were I started out, but which are def less fashionable now – guess its an age thing in my case. I have also just persuaded my good lady to get a hybrid for putting in miles on easier riding so interested in ideas.
Perhaps best for the OP to check what his visitor means by gravel riding. Eroded Scottish hill tracks might not be what she has in mind 🙂
Around Aviemore and Nethy Bridge would also be my first thought as we discovered a lot of the tracks no longer need a mountain bike.
Pattack I agree also has potential.
Glen Ogle can be made into a circular route by going through Glen Ample as it has an estate path running all the way through it – though do G Ample first and return G Ogle
This place is usually so dismissive of Glentrool I hadn’t considered it as an option, maybe need to reconsider
G Tilt, unless out and back to the bridge, would involve a fair bit of carrying on the Beinn a Ghlo circuit
G Finglas unless equipped with legs of steel and excellent brakes would be a bit masochistic on a cross bike in my view – tho I know others have done it.
Falkirk does now have the Wheel and the Helix and a (poorly) signposted circuit between them which makes for an entertaining few hours so not totally daft for a visitor – tho maybe not high on a must do list
Another option is the forest tracks from Aberfoyle – heading out through Milton to the south of L ard and then over towards L Chon. A lot of the forest has recently been cleared and there are some spectacular views. There is then potential for a loop around L Katrine and a return over by the L Drunkie tracks to take in the Trossachs