with regards to Yokomo G compound tires,
G2 means "G compound, 2 pairs"
The 139G has a more rounded profile than the 138, this should help in rolling resistance and wear rate I guess.
The G2 are definately not the hardest of the G series, if anything, they seem to be the "qualifier" tires as they didnt last that long with the kind of high surface temperature we run in here at the Equator.