The g-index is a variation of the h-index.
"The g-index g is the largest rank (where papers are arranged in decreasing order of the number of citations they received) such that the first g papers have (together) at least g2 citations" (Egghe, 2006).
Unlike the h-index, the g-index is strongly affected by highly-cited items.
Reference
Egghe, L. (2006) Theory and practice of the g-index. Scientometrics, 69(1): pp131-152.