For the purpose of counting generations you and the girl concerned are counted as generation one. Sapinda relationships are counted along patriarcial lines. For instance your patriarcial line through your father and your mother's patriarcial line through her father. Similarly for the girl too. It is not known whether the girl is your grandfather's sister's son's daughter or daughter's daughter. Anyway she does not come within your sapinda relationship. She is your second cross-cousin. If marrying second cousin is customary in your community, you can marry her.