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The only gravel sections I recall were due to road resurfacing work, which I was initially going to argue that this isn't something you can plan for -but on second thoughts guess that over the distance are likely to be encountered.
Spares-
Every race report I read mentioned sections of resurfacing.
There's also AC forums where people provide info about temporary issues:
https://www.adventurecycling.org/routes-and-maps/adventure-cycling-route-network/temporary-road-closures/ (https://forums.adventurecycling.org/index.php?PHPSESSID=d4peucm60qbep4o9bfmvni52d5&board=15.0)
and their twitter, where you could ask recent riders about roads:
https://twitter.com/acaroutesOr, just like accept that on 6700km of roads you might find some less than perfect conditions! :D
hippy
Sounds like a failure of planning if he didn't know about gravel sections given the route is out there, and he didn't test his brakes before the race...
@hippy, did you ever consider using your PX for this? If anyone could TT this distance...
Or ruled it out as too much faff fitting bags on, no-one's back / neck could stand 15+ days in TT position, did research and found gravel, etc.