It can be hit or miss. I would suggest you contact one of the taxi companies in advance and then again about 10 mins before arrival on your cell phone. That has always worked best for me at Gainesville or Toccoa. Here is a list of some companies you can try.
Taxi Azteca -429 Industrial Boulevard, Gainesville - (770) 297-8778
Latin America Cab Co - 628 Grove Street, Gainesville - (770) 539-9161
El Palmar Taxi -1783 Atlanta Road, Gainesville - (770) 534-3878
Taxi El Dorado -711 Industrial Blvd # B, Gainesville - (770) 297-6666
Taxi Los Potros -842 Main Street Southwest, Gainesville - (770) 534-9222
American Service Yellow Taxi -2101 Browns Bridge Rd, Gainesville - (770) 297-0255