There's not much info out there about gravels. Breeders seem to be buying visual highways since I'm guessing the breeder can't tell the difference between the gravels and YBs.
I've said it before, and I'll say it again here, if people are using such poor quality yellowbelly's that the gravels look similar, then maybe they should use a line of yb that is a little better quality

The problem with markers on some of incomplete dominant traits is the different lines present themselves differently.
My Haas line specter looks different from other lines I've seen, so giving the markers for specter just doesn't work so well, unless you want to list markers for different lines also.