Advisor–advisee relationships in academic genealogy offer opportunities to understand the contribution of a mentor in shaping the research community. In this paper, we adapt the bibliometric g-index to study the mentorship role of a researcher. We call the new index g_m-index. It has some important differences from the mentorship h-index or the h_m-index. We compute the values of the h_m and g_m indices for researchers indexed in the Mathematics Genealogy Project and the Academic Family Tree. We observe for the majority of researchers, these index values are zero, but in non-zero cases, sometimes, the g_m-index can be significantly higher than the h_m-index. Moreover, the g_m-index decays less rapidly to zero than the h_m-index. It appears the g_m-index can be used to discriminate between researchers with the same h_m-index. We study how these mentorship indices are correlated with other indicators of academic success, and how they are correlated across generations of researchers.