if you are correct that you and your relatives inherited 100% from your great-grandmother at the date of her death, then the basis would be fair market value at that date. The basis she got when she inherited it would be irrelevant. if your CPAs and lawyers can't answer this question find ones that will verify i'm correct.
if she died recently and the property is sold shortly in the future, unless there's been substantial appreciation between date of death and date sold, there should be virtually no gain. in many such situations there is in fact a loss because closing cost related to sale are not taken into account in determining fair market value but are taken into account in determining gain
