I have a train ticket to Blair Atholl have a few days pottering around on my own.

I don't know any options at all. I just bought a ticket there as I could head in a couple of directions. And don't know what would be doable on a cyclocross bike. Any suggestions for gravel grinding routes? From Blair Atholl, I could go into Cairngorms and Glen Tilt (is it possible on that bike?) or around Loch Tay/Rannoch/Ericht

Large parts of the classic route around Beinn a Ghlo would be doable on a CX - Glen Tilt as far as the Falls of Tarf and the track leading back from Fealar Lodge are 4WD track. The bits that are hike-a-bike on an MTB are going to be the same for a CX. The only bit that might be "interesting" is the section back from Loch Loch to the road, some will be OK, some won't.

How about the cycle route to dalwhinnie/laggan then general wades route to Ft Augustus then onto Fort William from there I have no clue

If you head in Loch Ericht direction, after the (ex-)Culra will be not much fun on a cx bike. Going the other way from Culra on the HT route via Loch Pattack to Laggan Wolftrax would be fine on Cx.
You could also head north from BA for about 10 miles on the cycletrack parallel to the A9 and go over the Gaick: 2km by the Loch tricky on cx but the rest fine.

Piemonster and I did a very CX loop from BA by heading towards Trinafour then up the back of Loch Bhac and down to Pitlochry. I can't get a GPX of it at the moment but could do at the weekend


https://www.strava.com/activities/921188884/

Lots of place names mentioned that I am clueless about, but that is a good start for me to hit the maps at the weekend. I do have a month to plan

The loop is kinda what I was after, so I just need to work out how to extend it. Don't bother with the gpx if you don't have it to hand as I'll make one with your strava as a guide

:-bd

The gaik will go on a CX bike - just one bit of single track but the rest is double. Come back by NCN7. Or get off at Dunkeld station, head up the track to the A822 and to Rumbling bridge and then over the forested and wind turbine covered hill (west of Craigvinean) to Aberfeldy (and back by NCN 7) From Dunkeld there is a network of tracks and trails to the NNE (ish) which will provide a days entertainment. Blair Atholl to Pitlochrie via the riverside paths would be a hoot on a CX bike. Or go up from Bridge of Tilt north west on the track over into Glen Bruar, head up past Bruar lodge to the start of the minigaig and either head back the way you came or do a lot of carrying over to Glen Tromie and then back to the Gaik....