Glad you sorted your travel. Public transport with bikes is a hassle.
I did it on a rigid mtb with 29er 2.6 inch tyres. When we got to the train station a couple of gravel riders were abandoning as they had split tyres. They said they had struggled coming down off the pass.
There was a lot of road on the last half but at no point did I think id have preferred it on a gravel bike, some of the descents were a lot of fun with the reassurance of bigger tyres.
I also needed the easier gearing on my mtb.
I really enjoyed it except the 2 and a half hours of pain after fort Augustus.
We too took extra food but managed to find enough on route but we were glad to have it just in case. The highlight was the post office cafe just after a big descent. Can’t remember the name but the food was amazing.
And the sawyer water filter got used more than any other trip I’ve ever done but probably because there’s a lot less livestock on the hills than I’m used to.