Financial services giant GFI Group, which has been a tenant at 55 Water St. since 2007, is more than doubling its space there. The firm is expanding by 64,993 square feet and also renewing 63,756 square feet. The rent at the 1972-vintage tower was $59 per square foot, reflecting the typical differential over prices in Midtown and at the World Trade Center. The additional space represents a consolidation by GFI from 199 Water St., where it had 70,000 square feet. GFI’s long-term commitment at 55